Troubleshooting

13 Troubleshooting

Understand how to find VirtualBox files and processes, how to collect debugging information, and identify and address common issues that can occur while working with Oracle VirtualBox.

Where Oracle VirtualBox Stores its Files

In Oracle VirtualBox, a virtual machine and its settings are described in a virtual machine settings file in XML format. In addition, most virtual machines have one or more virtual hard disks. These are typically represented by disk images, such as those in VDI format. The location of these files may vary, depending on the host operating system. See The VM Folder.

Global configuration data for Oracle VirtualBox is maintained in another location on the host. See Global Settings.

The VM Folder

By default, each virtual machine has a directory on your host computer where all the files of that machine are stored: the XML settings file, with a .vbox file extension, and its disk images. This is called the VM Folder.

By default, this machine folder is located in a common folder called VirtualBox VMs, which Oracle VirtualBox creates in the current system user's home directory. The location of this home directory depends on the conventions of the host operating system, as follows:

On Windows, this is the location returned by the SHGetFolderPath function of the Windows system library Shell32.dll, asking for the user profile. A typical location is C:\Users\username.
On Linux, macOS, and Oracle Solaris, this is generally taken from the environment variable $HOME, except for the user root where it is taken from the account database. This is a workaround for the frequent trouble caused by users using Oracle VirtualBox in combination with the tool sudo, which by default does not reset the environment variable $HOME.

A typical location on Linux and Oracle Solaris is /home/username and on macOS is /Users/username.

For simplicity, we abbreviate the location of the home directory as $HOME. Using that convention, the common folder for all virtual machines is $HOME/VirtualBox VMs.

As an example, when you create a virtual machine called Example VM, Oracle VirtualBox creates the following:

A VM folder: $HOME/VirtualBox VMs/Example VM/
In the VM folder, a settings file: Example VM.vbox
In the VM folder, a virtual disk image: Example VM.vdi.

This is the default layout if you use the Create New Virtual Machine workflow described in Creating a New Virtual Machine. Once you start working with the VM, additional files are added. Log files are in a subfolder called Logs, and if you have taken snapshots, they are in a Snapshots subfolder. For each VM, you can change the location of its snapshots folder in the VM settings.

To change the default VM folder:

In Oracle VirtualBox Manager, click File, Preferences.
In Preferences, click General and change the Default Machine Folder.

Alternatively, use the VBoxManage setproperty machinefolder command. See VBoxManage setproperty.

Global Settings

In addition to the files for the virtual machines, Oracle VirtualBox maintains global configuration data in the following directory:

Linux and Oracle Solaris:$HOME/.config/VirtualBox.
Windows:$HOME/.VirtualBox.
macOS:$HOME/Library/VirtualBox.

Oracle VirtualBox creates this configuration directory automatically, if necessary. You can specify an alternate configuration directory by either setting the VBOX_USER_HOME environment variable, or on Linux or Oracle Solaris by using the standard XDG_CONFIG_HOME variable. Since the global VirtualBox.xml settings file points to all other configuration files, this enables switching between several Oracle VirtualBox configurations.

In this configuration directory, Oracle VirtualBox stores its global settings file, an XML file called VirtualBox.xml. This file includes global configuration options and a list of registered virtual machines with pointers to their XML settings files.

Summary of Configuration Data Locations

The following table gives a brief overview of the configuration data locations on an Oracle VirtualBox host.

Table 13-1 Configuration File Locations

Setting	Location
Default machines folder	`$HOME/VirtualBox VMs`
Default disk image location	In each machine's folder
Machine settings file extension	`.vbox`
Media registry	Each machine settings file Media registration is done automatically when a storage medium is attached to a VM

Oracle VirtualBox XML Files

Oracle VirtualBox uses XML for both the machine settings files and the global configuration file, VirtualBox.xml.

All Oracle VirtualBox XML files are versioned. When a new settings file is created, for example because a new virtual machine is created, Oracle VirtualBox automatically uses the settings format of the current Oracle VirtualBox version. These files may not be readable if you downgrade to an earlier version of Oracle VirtualBox. However, when Oracle VirtualBox encounters a settings file from an earlier version, such as after upgrading Oracle VirtualBox, it attempts to preserve the settings format as much as possible. It will only silently upgrade the settings format if the current settings cannot be expressed in the old format, for example because you enabled a feature that was not present in an earlier version of Oracle VirtualBox.

In such cases, Oracle VirtualBox backs up the old settings file in the virtual machine's configuration directory. If you need to go back to the earlier version of Oracle VirtualBox, then you will need to manually copy these backup files back.

We intentionally do not document the specifications of the Oracle VirtualBox XML files, as we must reserve the right to modify them in the future. We therefore strongly suggest that you do not edit these files manually. Oracle VirtualBox provides complete access to its configuration data through its the VBoxManage command line tool, see VBoxManage and its API, see Oracle VirtualBox Programming Interfaces.

Oracle VirtualBox Host Processes

Oracle VirtualBox was designed to be modular and flexible. When the Oracle VirtualBox graphical user interface (GUI) is opened and a VM is started, at least the following three processes are running:

VBoxSVC, the Oracle VirtualBox service process which always runs in the background. This process is started automatically by the first Oracle VirtualBox client process and exits a short time after the last client exits.

Note:

When we refer to clients here, we mean the local clients of a particular VBoxSVC server process, not clients in a network. Oracle VirtualBox employs its own client/server design to allow its processes to cooperate, but all these processes run under the same user account on the host operating system, and this is totally transparent to the user.

The first Oracle VirtualBox service can be Oracle VirtualBox Manager, VBoxManage, VBoxHeadless, the web service amongst others. The service is responsible for bookkeeping, maintaining the state of all VMs, and for providing communication between Oracle VirtualBox components. This communication is implemented using COM/XPCOM. Whenever an Oracle VirtualBox process is started, it requests access to the COM server and Windows automatically starts the process. You should never start the process directly.

When the last process disconnects from the COM server, it will stop itself after a few seconds. The Oracle VirtualBox configuration XML files are maintained and owned by the COM server and the files are locked whenever the server runs.

In some cases, such as when a virtual machine is closed unexpectedly, the COM server will not notice that the client is disconnected and stay active for a longer period of 10 minutes or so, keeping the configuration files locked. In other rare cases the COM server might experience an internal error and subsequently other processes fail to initialize it. In these situations, use the Windows task manager to stop the process VBoxSVC.exe.
The GUI process, VirtualBoxVM, a client application based on the cross-platform Qt library. When started without the --startvm option, this application acts as Oracle VirtualBox Manager, displaying the VMs and their settings. It then communicates settings and state changes to VBoxSVC and also reflects changes effected through other means, such as the VBoxManage command.
If the VirtualBoxVM client application is started with the --startvm argument, it loads the VMM library which includes the actual hypervisor and then runs a virtual machine and provides the input and output for the guest.

Any Oracle VirtualBox front end, or client, will communicate with the service process and can both control and reflect the current state. For example, either the VM selector or the VM window or VBoxManage can be used to pause the running VM, and other components will always reflect the changed state.

The Oracle VirtualBox GUI application, called Oracle VirtualBox Manager, is only one of several available front ends, or clients. The complete list shipped with Oracle VirtualBox is as follows:

VirtualBoxVM: The Qt front end implementing Oracle VirtualBox Manager and running VMs.
VBoxManage: A less user-friendly but more powerful alternative. See VBoxManage.
VBoxHeadless: A VM front end which does not directly provide any video output and keyboard or mouse input, but enables redirection through the VirtualBox Remote Desktop Extension. See VBoxHeadless, the Remote Desktop Server.
vboxwebsrv: The Oracle VirtualBox web service process which enables control of an Oracle VirtualBox host remotely. This is described in detail in the Oracle VirtualBox Software Development Kit (SDK) reference. See Oracle VirtualBox Programming Interfaces.
The Oracle VirtualBox Python shell: A Python alternative to VBoxManage. This is also described in the SDK reference.

Internally, Oracle VirtualBox consists of many components. You may encounter these when analyzing Oracle VirtualBox internal error messages or log files. These include the following:

IPRT: A portable runtime library which abstracts file access, threading, and string manipulation. Whenever Oracle VirtualBox accesses host operating features, it does so through this library for cross-platform portability.
VMM (Virtual Machine Monitor): The heart of the hypervisor.
EM (Execution Manager): Controls execution of guest code.
TRPM (Trap Manager): Intercepts and processes guest traps and exceptions.
HM (Hardware Acceleration Manager): Provides support for VT-x and AMD-V.
GIM (Guest Interface Manager): Provides support for various paravirtualization interfaces to the guest.
PDM (Pluggable Device Manager): An abstract interface between the VMM and emulated devices which separates device implementations from VMM internals and makes it easy to add new emulated devices. Through PDM, third-party developers can add new virtual devices to Oracle VirtualBox without having to change Oracle VirtualBox itself.
PGM (Page Manager): A component that controls guest paging.
TM (Time Manager): Handles timers and all aspects of time inside guests.
CFGM (Configuration Manager): Provides a tree structure which holds configuration settings for the VM and all emulated devices.
SSM (Saved State Manager): Saves and loads VM state.
VUSB (Virtual USB): A USB layer which separates emulated USB controllers from the controllers on the host and from USB devices. This component also enables remote USB.
DBGF (Debug Facility): A built-in VM debugger.
Oracle VirtualBox emulates a number of devices to provide the hardware environment that various guests need. Most of these are standard devices found in many PC compatible machines and widely supported by guest operating systems. For network and storage devices in particular, there are several options for the emulated devices to access the underlying hardware. These devices are managed by PDM.
Guest Additions for various guest operating systems. This is code that is installed from within a virtual machine. See Guest Additions.
The "Main" component is special. It ties all the above bits together and is the only public API that Oracle VirtualBox provides. All the client processes listed above use only this API and never access the hypervisor components directly. As a result, third-party applications that use the Oracle VirtualBox Main API can rely on the fact that it is always well-tested and that all capabilities of Oracle VirtualBox are fully exposed. It is this API that is described in the Oracle VirtualBox SDK. See Oracle VirtualBox Programming Interfaces.

Oracle VirtualBox Guest Processes

Oracle VirtualBox VMs also run VirtualBox processes.

If a particular desktop service is not working correctly, it is worth checking that the process that should provide it is running.

VBoxClient provides desktop services in guests running the X11 window system such as Oracle Solaris and Linux. This process runs under the ID of the user who started the desktop session and is automatically started using the following command lines when your X11 user session is started if you are using a common desktop environment such as Gnome or KDE.
```
$ VBoxClient --clipboard
$ VBoxClient --display
$ VBoxClient --seamless
```
The VBoxClient processes create files in the user's home directory with names of the form .vboxclient-*.pid when they are running in order to prevent a given service from being started twice. It can happen due to misconfiguration that these files are created owned by root and not deleted when the services are stopped, which will prevent them from being started in future sessions. If the services cannot be started, you may want to check whether these files still exist.

Categorizing and Isolating Problems

Usually a virtual machine behaves like a physical machine. Any problems that a physical machine would encounter, a virtual machine will encounter as well. If, for example, Internet connectivity is lost due to external issues, virtual machines will be affected just as much as physical ones.

If a true Oracle VirtualBox problem is encountered, it helps to categorize and isolate the problem first. Here are some of the questions that should be answered when reporting a problem:

Is the problem specific to a certain guest OS? Or a specific release of a guest OS? Especially with Linux guest related problems, the issue may be specific to a certain distribution and version of Linux.
Is the problem specific to a certain host OS? Problems are usually not host OS specific, because most of the Oracle VirtualBox code base is shared across all supported platforms. However in the areas of networking and USB support, there are significant differences between host platforms. Some GUI related issues are also host specific.
Is the problem specific to certain host hardware? This category of issues is typically related to the host CPU. Because of significant differences between VT-x and AMD-V, problems may be specific to one or the other technology. The exact CPU model may also make a difference because different CPUs support different features, which may affect certain aspects of guest CPU operation.
Is the problem specific to guest SMP? That is, is it related to the number of virtual CPUs (VCPUs) in the guest? Using more than one CPU usually significantly affects the internal operation of a guest OS.
Is the problem specific to the Guest Additions? In some cases, this is obvious, such as a shared folders problem. In other cases, such as display problems, it may be less obvious. If the problem is Guest Additions specific, is it also specific to a certain version of the Guest Additions?
Is the problem specific to a certain environment? Some problems are related to a particular environment external to the VM. This usually involves network setup. Certain configurations of external servers such as DHCP or PXE may expose problems which do not occur with other, similar servers.
Is the problem a regression? Knowing that an issue is a regression usually makes it significantly easier to find the solution. In this case, it is crucial to know which version is affected and which is not.
Are the processes running that should be? See Oracle VirtualBox Host Processes.

Collecting Debugging Information

For problem determination, it is often important to collect debugging information which can be analyzed by Oracle VirtualBox support. This section contains information about what kind of information can be obtained.

Every time Oracle VirtualBox starts up a VM, a so-called release log file is created, containing lots of information about the VM configuration and runtime events. The log file is called VBox.log and resides in the VM log file folder, which is $HOME/VirtualBox VMs/VM-name/Logs by default.

When starting a VM, the configuration file of the last run will be renamed to .1, up to .3. Sometimes when there is a problem, it is useful to have a look at the logs. You are also required to supply the corresponding log file when requesting support for Oracle VirtualBox.

For convenience, for each virtual machine, Oracle VirtualBox Manager can show these logs in a window. Select a virtual machine from the machine list on the left and click Logs in the machine tools menu.

The release log file, VBox.log, contains a wealth of diagnostic information, such as Host OS type and version, Oracle VirtualBox version and build. It also includes a complete dump of the guest's configuration (CFGM), detailed information about the host CPU type and supported features, whether hardware virtualization is enabled, information about VT-x/AMD-V setup, state transitions (such as creating, running, paused, stopping), guest BIOS messages, Guest Additions messages, device-specific log entries and, at the end of execution, final guest state and condensed statistics.

In case of crashes, it is very important to collect crash dumps. This is true for both host and guest crashes. For information about enabling core dumps on Linux, Oracle Solaris, and macOS systems, refer to the following core dump article on the Oracle VirtualBox website:

http://www.virtualbox.org/wiki/Core_dump.

You can also use VBoxManage debugvm to create a dump of a complete virtual machine. See VBoxManage debugvm.

For network related problems, it is often helpful to capture a trace of network traffic. If the traffic is routed through an adapter on the host, it is possible to use Wireshark or a similar tool to capture the traffic there. However, this often also includes a lot of traffic unrelated to the VM.

Oracle VirtualBox provides an ability to capture network traffic only on a specific VM's network adapter. Refer to the following network tracing article on the Oracle VirtualBox website for information on enabling this capture:

http://www.virtualbox.org/wiki/Network_tips.

The trace files created by Oracle VirtualBox are in .pcap format and can be easily analyzed with Wireshark.

Using the VBoxBugReport Command to Collect Debug Information Automatically

The VBoxBugReport command is used to collect debug information automatically for an Oracle VirtualBox installation. This command can be useful when you need to gather information to send to Oracle Support.

The following examples show how to use VBoxBugReport.

By default, the command collects VBoxSVC process logs, device settings, and global configuration data for an Oracle VirtualBox host.

$ VBoxBugReport
  ...
  0% - collecting VBoxSVC.log.10...
  7% - collecting VBoxSVC.log.9...
  ...
 64% - collecting VBoxSVC.log.1...
 71% - collecting VBoxSVC.log...
 78% - collecting VirtualBox.xml...
 85% - collecting HostUsbDevices...
 92% - collecting HostUsbFilters...
100% - compressing...

Report was written to '2019-03-26-13-32-02-bugreport.tgz'

The results are saved as a compressed tar file archive in the same directory where the command is run.

To specify a different output file location:

$ VBoxBugReport --output ~/debug/bug004.tgz

To output all debug information to a single text file, rather than a tgz file:

$ VBoxBugReport --text

To collect information for a specific VM, called Windows_10:

$ VBoxBugReport Windows_10

This command collects machine settings, guest properties, and log files for the specified VM. Global configuration information for the host is also included.

To collect information for several VMs, called Windows_7, Windows_8, and Windows_10:

$ VBoxBugReport Windows_7 Windows_8 Windows_10

To collect information for all VMs:

$ VBoxBugReport --all

To show a full list of the available command options, run VBoxBugReport --help.

The Built-In VM Debugger

Oracle VirtualBox includes a built-in VM debugger, which advanced users may find useful. This debugger enables you to examine and, to some extent, control the VM state.

Caution:

Use the VM debugger at your own risk. There is no support for it, and the following documentation is only made available for advanced users with a very high level of familiarity with the x86/AMD64 machine instruction set, as well as detailed knowledge of the PC architecture. A degree of familiarity with the internals of the guest OS in question may also be very helpful.

The VM debugger is available in all regular production versions of Oracle VirtualBox, but it is disabled by default because the average user will have little use for it. There are two ways to access the debugger:

Using a debugger console window displayed alongside the VM
Using the telnet protocol on port 5000

The debugger can be enabled in the following ways:

Start the VM directly using VirtualBoxVM --startvm, with an additional --dbg, --debug, or --debug-command-line argument. See the VirtualBoxVM --help command usage help for details.
Set the VBOX_GUI_DBG_ENABLED or VBOX_GUI_DBG_AUTO_SHOW environment variable to true before launching the Oracle VirtualBox process. Setting these variables, only their presence is checked, is effective even when the first Oracle VirtualBox process is the VM selector window. VMs subsequently launched from the selector will have the debugger enabled.
Set the GUI/Dbg/Enabled extra data item to true before launching the VM. This can be set globally or on a per VM basis.

A new Debug menu entry is added to the Oracle VirtualBox application. This menu enables the user to open the debugger console.

The VM debugger command syntax is loosely modeled on Microsoft and IBM debuggers used on DOS, OS/2, and Windows. Users familiar with symdeb, CodeView, or the OS/2 kernel debugger will find the Oracle VirtualBox VM debugger familiar.

The most important command is help. This will print brief usage help for all debugger commands. The set of commands supported by the VM debugger changes frequently and the help command is always up-to-date.

A brief summary of frequently used commands is as follows:

stop: Stops the VM execution and enables single stepping
g: Continue VM execution
t: Single step an instruction
rg, rh, and r: Print the guest, hypervisor, and current registers
kg, kh, and k: Print the guest, hypervisor, and current call stack
da, db, dw, dd, dq: Print memory contents as ASCII, bytes, words, dwords, and qwords
u: Unassemble memory
dg: Print the guest's GDT
di: Print the guest's IDT
dl: Print the guest's LDT
dt: Print the guest's TSS
dp*: Print the guest's page table structures
bp and br: Set a normal and recompiler breakpoint
bl: List breakpoints
bc: Clear a breakpoint
writecore: Write a VM core file to disk. See VM Core Format

See the built-in help for other available commands.

The VM debugger supports symbolic debugging, although symbols for guest code are often not available. For Oracle Solaris guests, the detect command automatically determines the guest OS version and locates kernel symbols in guest's memory. Symbolic debugging is then available. For Linux guests, the detect commands also determines the guest OS version, but there are no symbols in the guest's memory. Kernel symbols are available in the file /proc/kallsyms on Linux guests. This file must be copied to the host, for example using scp. The loadmap debugger command can be used to make the symbol information available to the VM debugger. Note that the kallsyms file contains the symbols for the currently loaded modules. If the guest's configuration changes, the symbols will change as well and must be updated.

For all guests, a simple way to verify that the correct symbols are loaded is the k command. The guest is normally idling and it should be clear from the symbolic information that the guest operating system's idle loop is being executed.

Another group of debugger commands is the set of info commands. Running info help provides complete usage information. The information commands provide ad-hoc data pertinent to various emulated devices and aspects of the VMM. There is no general guideline for using the info commands, the right command to use depends entirely on the problem being investigated. Some of the info commands are as follows:

cfgm: Print a branch of the configuration tree
cpuid: Display the guest CPUID leaves
ioport: Print registered I/O port ranges
mmio: Print registered MMIO ranges
mode: Print the current paging mode
pit: Print the i8254 PIT state
pic: Print the i8259A PIC state
ohci, ehci, xhci: Print a subset of the OHCI, EHCI, and xHCI USB controller state
pcnet0: Print the PCnet state
vgatext: Print the contents of the VGA framebuffer formatted as standard text mode
timers: Print all VM timers

The output of the info commands generally requires in-depth knowledge of the emulated device or Oracle VirtualBox VMM internals. However, when used properly, the information provided can be very valuable.

VM Core Format

Oracle VirtualBox uses the 64-bit ELF format for its VM core files created by VBoxManage debugvm, see VBoxManage debugvm. The VM core file contain the memory and CPU dumps of the VM and can be useful for debugging your guest OS. The 64-bit ELF object format specification can be obtained at:

http://downloads.openwatcom.org/ftp/devel/docs/elf-64-gen.pdf.

The overall layout of the VM core format is as follows:

[ ELF 64 Header]
[ Program Header, type PT_NOTE ]
  → offset to COREDESCRIPTOR
[ Program Header, type PT_LOAD ] - one for each contiguous physical memory range
  → Memory offset of range
  → File offset
[ Note Header, type NT_VBOXCORE ]
[ COREDESCRIPTOR ]
  → Magic
  → VM core file version
  → VBox version
  → Number of vCPUs etc.
[ Note Header, type NT_VBOXCPU ] - one for each vCPU
[ vCPU 1 Note Header ]
  [ DBGFCORECPU - vCPU 1 dump ]
[ Additional Notes + Data ] - currently unused
[ Memory dump ]

The memory descriptors contain physical addresses relative to the guest and not virtual addresses. Regions of memory such as MMIO regions are not included in the core file.

The relevant data structures and definitions can be found in the Oracle VirtualBox sources under the following header files: include/VBox/dbgfcorefmt.h, include/iprt/x86.h and src/VBox/Runtime/include/internal/ldrELFCommon.h.

The VM core file can be inspected using elfdump and GNU readelf or other similar utilities.

Running Legacy Guest OSs

Older Operating Systems that no longer qualify for Oracle support may work in Oracle VirtualBox, but may need some additional setup and will have limitations. Here are some suggestions and workarounds for common problems.

Graphics and Mouse Integration

In older Linux and Oracle Solaris guests, Oracle VirtualBox graphics and mouse integration goes through the X Window System. Oracle VirtualBox can use the X.Org variant of the system, or XFree86 version 4.3 which is identical to the first X.Org release. During the installation process, the X.Org display server will be set up to use the graphics and mouse drivers that come with the Guest Additions.

After installing the Guest Additions into a fresh installation of most Linux distributions or Oracle Solaris systems, the guest's graphics mode will change to fit the size of the Oracle VirtualBox window on the host when it is resized. You can also ask the guest system to switch to a particular resolution by sending a video mode hint using the VBoxManage tool.

Multiple guest monitors are supported in guests using the X.Org server version 1.3, which is part of release 7.3 of the X Window System version 11, or a later version. The layout of the guest screens can be adjusted as needed using the tools which come with the guest operating system.

The Oracle VirtualBox Guest Additions includes drivers for X.Org. You should not need to change these. By default these drivers are in the following directory:

/opt/VBoxGuestAdditions-version/other/

The correct versions for the X server are symbolically linked into the X.Org driver directories.

For graphics integration to work correctly, the X server must load the vboxvideo driver. Many recent X server versions look for it automatically if they see that they are running in Oracle VirtualBox. For an optimal user experience, the guest kernel drivers must be loaded and the Guest Additions tool VBoxClient must be running as a client in the X session.

For mouse integration to work correctly, the guest kernel drivers must be loaded. In addition, for legacy X servers the correct vboxmouse driver must be loaded and associated with /dev/mouse or /dev/psaux. For most guests, a driver for a PS/2 mouse must be loaded and the correct vboxmouse driver must be associated with /dev/vboxguest.

The Oracle VirtualBox guest graphics driver can use any graphics configuration for which the virtual resolution fits into the virtual video memory allocated to the virtual machine, minus a small amount used by the guest driver, as described in Display Settings. The driver will offer a range of standard modes at least up to the default guest resolution for all active guest monitors. The default mode can be changed by setting the output property VBOX_MODE to "<width>x<height>" for any guest monitor. When VBoxClient and the kernel drivers are active this is done automatically when the host requests a mode change. The driver for older versions can only receive new modes by querying the host for requests at regular intervals.

With legacy X Servers before version 1.3, you can also add your own modes to the X server configuration file. Add them to the "Modes" list in the "Display" subsection of the "Screen" section. For example, the following section has a custom 2048x800 resolution mode added:

Section "Screen"
        Identifier    "Default Screen"
        Device        "VirtualBox graphics card"
        Monitor       "Generic Monitor"
        DefaultDepth  24
        SubSection "Display"
                Depth         24
                Modes         "2048x800" "800x600" "640x480"
        EndSubSection
EndSection

Guest OSs With no ATAPI CD-ROM Support May Fail to Boot

Some guest operating systems predating ATAPI CD-ROMs may exhibit long delays or entirely fail to boot in certain configurations. This is most likely to happen when an IDE/ATAPI CD-ROM exists alone on a primary or secondary IDE channel. For example, OS/2 1.21 fails to boot with an error message referencing COUNTRY.SYS, and OS/2 1.3 experiences long boot delays.

Disable the emulated IDE/ATAPI CD-ROM. The guest OS can't use this device anyway.

Windows 2000 Installation Failures

When installing Windows 2000 guests, you might run into one of the following issues:

Installation reboots, usually during component registration.
Installation fills the whole hard disk with empty log files.
Installation complains about a failure installing msgina.dll.

These problems are all caused by a bug in the hard disk driver of Windows 2000. After issuing a hard disk request, there is a race condition in the Windows driver code which leads to corruption if the operation completes too fast. For example, the hardware interrupt from the IDE controller arrives too soon. With physical hardware, there is a guaranteed delay in most systems so the problem is usually hidden there. However, it should be possible to also reproduce it on physical hardware. In a virtual environment, it is possible for the operation to be done immediately, especially on very fast systems with multiple CPUs, and the interrupt is signaled sooner than on a physical system. The solution is to introduce an artificial delay before delivering such interrupts. This delay can be configured for a VM using the following command:

$ VBoxManage setextradata VM-name "VBoxInternal/Devices/piix3ide/0/Config/IRQDelay" 1

This sets the delay to one millisecond. In case this does not help, increase it to a value between 1 and 5 milliseconds. Please note that this slows down disk performance. After installation, you should be able to remove the key, or set it to 0.

Windows 3.x Limited to 64 MB RAM

Windows 3.x guests are typically limited to 64 MB RAM, even if a VM is assigned much more memory. While Windows 3.1 is theoretically capable of using up to 512 MB RAM, it only uses memory available through the XMS interface. Versions of HIMEM.SYS, the Microsoft XMS manager, shipped with MS-DOS and Microsoft Windows 3.x can only use up to 64 MB on standard PCs.

This is a known HIMEM.SYS limitation. Windows 3.1 memory limits are described in detail in Microsoft Knowledge base article KB 84388.

It is possible for Windows 3.x guests to utilize more than 64 MB RAM if a different XMS provider is used. That could be a newer HIMEM.SYS version, such as that shipped with Windows 98, or a more capable third-party memory manager, such as QEMM.

No Networking in Windows Vista Guests

With Windows Vista, Microsoft dropped support for the AMD PCNet card that legacy versions of Oracle VirtualBox used to provide as the default virtual network card. For Windows Vista guests, Oracle VirtualBox now uses an Intel E1000 card by default.

If, for some reason, you still want to use the AMD card, you need to download the PCNet driver from the AMD website. This driver is available for 32-bit Windows only. You can transfer it into the virtual machine using a shared folder. See Shared Folders.

No USB 3.0 Support in Windows 7 Guests

If a Windows 7 or Windows Server 2008 R2 guest is configured for USB 3.0 (xHCI) support, the guest OS will not have any USB support at all. This happens because Windows 7 predates USB 3.0 and therefore does not ship with any xHCI drivers. Microsoft also does not offer any vendor-provided xHCI drivers through Windows Update.

To solve this problem, download and install the Intel xHCI driver in the guest. Intel offers the driver as the USB 3.0 eXtensible Host Controller (xHCI) driver for Intel 7 Series/C216 chipsets.

Note that the driver only supports Windows 7 and Windows Server 2008 R2. The driver package includes support for both 32-bit and 64-bit OS variants.

Windows Fails After Changing VM Configuration

Changing certain virtual machine settings can cause Windows guests to fail during start up with a bluescreen. This may happen if you change VM settings after installing Windows, or if you copy a disk image with an already installed Windows to a newly created VM which has settings that differ from the original machine.

This applies in particular to the following settings:

The ACPI and I/O APIC settings should never be changed after installing Windows. Depending on the presence of these hardware features, the Windows installation program chooses special kernel and device driver versions and will fail to startup should these hardware features be removed. Enabling them for a Windows VM which was installed without them does not cause any harm. However, Windows will not use these features in this case.
Changing the storage controller hardware will cause bootup failures. This might also apply to you if you copy a disk image from an older version of Oracle VirtualBox to a new virtual machine. The default subtype of IDE controller hardware used by Oracle VirtualBox is PIIX4. Make sure that the storage controller settings are identical.

IDE/SATA Errors on Guest OSs

Occasionally, some host file systems provide very poor writing performance and as a consequence cause the guest to time out IDE/SATA commands. This is normal behavior and should normally cause no real problems, as the guest should repeat commands that have timed out. However, guests such as some Linux versions have severe problems if a write to an image file takes longer than about 15 seconds. Some file systems however require more than a minute to complete a single write, if the host cache contains a large amount of data that needs to be written.

The result of this problem is that the guest can no longer access its files during large write or copying operations, usually leading to an immediate hang of the guest.

To work around this problem, it is possible to flush the image file after a certain amount of data has been written. This interval is normally infinite, but can be configured individually for each disk of a VM.

For IDE disks use the following command:

VBoxManage setextradata VM-name
"VBoxInternal/Devices/piix3ide/0/LUN#[x]/Config/FlushInterval" [b]

For SATA disks use the following command:

VBoxManage setextradata VM-name
"VBoxInternal/Devices/ahci/0/LUN#[x]/Config/FlushInterval" [b]

[x] specifies the disk. For IDE, 0 represents device 0 on the primary channel, 1 represents device 1 on the primary channel, 2 represents device 0 on the secondary channel, and 3 represents device 1 on the secondary channel. For SATA, use values between 0 and 29. This configuration option applies to disks only. Do not use this option for CD or DVD drives.

The unit of the interval ([b]) is the number of bytes written since the last flush. The value for it must be selected so that the occasional long write delays do not occur. Since the proper flush interval depends on the performance of the host and the host filesystem, finding the optimal value that makes the problem disappear requires some experimentation. Values between 1000000 and 10000000 (1 to 10 megabytes) are a good starting point. Decreasing the interval both decreases the probability of the problem and the write performance of the guest. Setting the value unnecessarily low will cost performance without providing any benefits. An interval of 1 will cause a flush for each write operation and should solve the problem, but with a severe write performance penalty.

A value of 0 for [b] is treated as an infinite flush interval, effectively disabling this workaround. Removing the extra data key by specifying no value for [b] has the same effect.

Buggy Linux 2.6 Kernel Versions

The following bugs in Linux kernels prevent them from executing correctly in Oracle VirtualBox, causing VM boot crashes:

The Linux kernel version 2.6.18, and some 2.6.17 versions, introduced a race condition that can cause boot failures in Oracle VirtualBox. Use kernel version 2.6.19 or later.
With hardware virtualization and the I/O APIC enabled, kernels before 2.6.24-rc6 may panic on boot with the following message:
```
Kernel panic - not syncing: IO-APIC + timer doesn't work!  Boot with
apic=debug and send a report.  Then try booting with the 'noapic' option
```
If you see this message, either disable hardware virtualization or the I/O APIC as described in System Settings, or upgrade the guest to a newer kernel.

See this message for details about the kernel fix.

Crash on Start Caused by Kernel Bugs

The version of the Linux kernel supplied by default in SUSE and openSUSE 10.2, Ubuntu 6.10 (all versions) and Ubuntu 6.06 (server edition) contains a bug that can cause it to crash during startup when it is run in a virtual machine.

Linux Guests May Cause a High CPU load

Some legacy Linux guests may cause a high CPU load even if the guest system appears to be idle. This can be caused by a high timer frequency of the guest kernel. Some older Linux distributions ship a Linux kernel configured for a timer frequency of 1000Hz. We recommend recompiling the guest kernel and selecting a timer frequency of 100Hz.

Linux kernels shipped with Red Hat Enterprise Linux, as well as kernels of related Linux distributions, such as CentOS and Oracle Linux, support a kernel parameter divider=N. Hence, such kernels support a lower timer frequency without recompilation. Add the kernel parameter divider=10 to select a guest kernel timer frequency of 100Hz.

Certain Oracle Solaris 10 Releases May Take a Long Time to Boot with SMP

When using more than one CPU, Oracle Solaris 10 10/08, and Oracle Solaris 10 5/09 may take a long time to boot and may print warnings on the system console regarding failures to read from disk. This is a bug in Oracle Solaris 10 that affects specific physical and virtual configurations. It is caused by trying to read microcode updates from the boot disk when the disk interrupt is reassigned to a not yet fully initialized secondary CPU. Disk reads will time out and fail, triggering delays of about 45 seconds, and warnings.

The recommended solution is upgrading to at least Oracle Solaris 10 10/09, which includes a fix for this problem. Alternative solutions include restricting the number of virtual CPUs to one or possibly using a different storage controller.

Older Solaris Releases Do Not Work with E1000 Ethernet

Solaris releases before Solaris 10 1/06, including Solaris 9, Solaris 10 1/05 (GA), and Solaris 10 3/05 (HW2), are unable to communicate through the Intel E1000 card. The Solaris e1000g driver does not enable PCI bus mastering for the network adapter and is therefore unable to send and receive data. This problem appears to be specific to the e1000g driver and does not reflect general Solaris driver behavior.

The AMD PCnet emulation (using the Solaris pcn driver) can be used instead of Intel E1000. Solaris 10 1/06 (U1) and later releases do not have this problem and work with the emulated E1000 ethernet controller.

Known Issues

The following section describes known problems with this release of Oracle VirtualBox. Unless marked otherwise, these issues are planned to be fixed in later releases.

General Issues

Responding to Guest IDE/SATA Flush Requests

If required, the virtual disk images can be flushed when the guest issues the IDE FLUSH CACHE command. Normally these requests are ignored for improved performance. The parameters below are only accepted for disk drives. They must not be set for DVD drives.

To enable flushing for IDE disks, issue the following command:

$ VBoxManage setextradata VM-name "VBoxInternal/Devices/piix3ide/0/LUN#[x]/Config/IgnoreFlush" 0

[x] specifies the disk. Enter 0 for device 0 on the primary channel, 1 for device 1 on the primary channel, 2 for device 0 on the secondary channel, or 3 for device 1 on the secondary channel.

To enable flushing for SATA disks, issue the following command:

$ VBoxManage setextradata VM-name "VBoxInternal/Devices/ahci/0/LUN#[x]/Config/IgnoreFlush" 0

The value [x] that selects the disk can be a value between 0 and 29.

Note that this does not affect the flushes performed according to the configuration described in IDE/SATA Errors on Guest OSs. Restoring the default of ignoring flush commands is possible by setting the value to 1 or by removing the key.

Performance Variation with Frequency Boosting

Many multicore processors support some form of frequency boosting, which means that if only one core is utilized, it can run possibly 50% faster or even more than the rated CPU frequency. This causes measured performance to vary somewhat as a function of the momentary overall system load. The exact behavior depends strongly on the specific processor model.

As a consequence, benchmarking on systems which utilize frequency boosting may produce unstable and nonrepeatable results. This is especially true if benchmark runs are short, of the order of seconds. To obtain stable results, benchmarks must be run over longer periods of time and with a constant system load apart from the VM being tested.

Frequency Scaling Effect on CPU Usage

On some hardware platforms and operating systems, CPU frequency scaling may cause CPU usage reporting to be highly misleading. This happens in situations when the host CPU load is significant but not heavy, such as between 15% to 30% of the maximum.

Most operating systems determine CPU usage in terms of time spent, measuring for example how many nanoseconds the systems or a process was active within one second. However, in order to save energy, systems can significantly scale down CPU speed when the system is not fully loaded. When the CPU is running at for example one half of its maximum speed, the same number of instructions will take roughly twice as long to execute compared to running at full speed.

Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it would have been had the CPU been running at full speed. The effect can be observed both on the host OS and in a guest OS.

Inaccurate Windows CPU Usage Reporting

CPU usage reporting tools which come with Windows, such as Task Manager or Resource Monitor, do not take the time spent processing hardware interrupts into account. If the interrupt load is heavy, with thousands of interrupts per second, CPU usage may be significantly underreported.

This problem affects Windows as both host and guest OS. Sysinternals tools, such as Process Explorer, do not suffer from this problem.

Poor Performance Caused by Host Power Management

On some hardware platforms and operating systems, virtualization performance is negatively affected by host CPU power management. The symptoms may be choppy audio in the guest or erratic guest clock behavior.

Some of the problems may be caused by firmware or host operating system bugs. Therefore, updating the firmware and applying operating system fixes is recommended.

For optimal virtualization performance, the C1E power state support in the system's BIOS should be disabled, if such a setting is available. Not all systems support the C1E power state. On Intel systems, the Intel C State setting should be disabled. Disabling other power management settings may also improve performance. However, a balance between performance and power consumption must always be considered.

Guest SMP (Multiprocessor) Limitations

Details

Poor performance with 32-bit guests on AMD CPUs. This affects Windows and Oracle Solaris guests, and possibly also some Linux kernel revisions. Partially solved for 32-bit Windows NT, 2000, XP, and 2003 guests.
Poor performance with 32-bit guests on certain Intel CPU models that do not include virtual APIC hardware optimization support. This affects Windows and Oracle Solaris guests, and possibly also some Linux kernel revisions. Partially solved for 32-bit Windows NT, 2000, XP, and 2003 guests.

Workaround

We're working on a resolution.

Ensure you have Guest Additions installed.

Can't Compact VMDK Virtual Disk Images

Details: The VBoxManage modifymedium --compact command is currently only implemented for VDI and VHD files. You can't use this command for VMDK files.
Workaround: At the moment the only way to optimize the size of a virtual disk image in VMDK is to clone the image and then use the cloned image in the VM configuration.

Preserving the Aspect Ratio in Scale Mode Not Available on All Hosts

Details: Preserving the aspect ratio in scale mode works only on Windows hosts and on macOS hosts.
Workaround: No workaround exists.

Drag Limitations

Details: On Linux hosts and guests, programs can query for dragged data while the drag operation is still in progress. For example, on LXDE using the PCManFM file manager. This feature does not work with Oracle VirtualBox.
Workaround: A different file manager, such as Nautilus, can be used to force programs to wait until a drag operation is complete before querying the data.

Guest OS Won't Run Using ICH9

Details

VMs using the ICH9 chipset will not run if they are running very old OSs that have no IO-APIC support, or any VM with IO-APIC disabled in the config. This is because interrupts coming from PCI devices can't be delivered.

The interrupt delivery to the legacy PIC interrupt controller is not implemented for ICH9, just the delivery to IO-APIC.

Workaround

Use the PIIX3 chipset for the VM.

Windows Guests

How to Record Bluescreen Information from Windows Guests

When Windows guests run into a kernel crash, they display a bluescreen error. Depending on how Windows is configured, the information will remain on the screen until the machine is restarted or it will reboot automatically. During installation, Windows is usually configured to reboot automatically. With automatic reboots, there is no chance to record the bluescreen information which might be important for problem determination.

Oracle VirtualBox provides a method of halting a guest when it wants to perform a reset. In order to enable this feature, use the following command:

$ VBoxManage setextradata VM-name "VBoxInternal/PDM/HaltOnReset" 1

Windows Guests Cause a High CPU Load

Several background applications in Windows guests, especially virus scanners, are known to increase the CPU load notably even if the guest appears to be idle. Deactivating virus scanners in VMs, where possible, can reduce this problem.

Windows Updates, especially on Windows 10 and Windows 11 guests, can also cause a temporary increase in guest CPU usage and storage activity. Modern Windows versions do a lot of updating in the background to deploy new features and security fixes.

Long Delays When Accessing Shared Folders

The performance for accesses to shared folders from a Windows guest might be decreased due to delays during the resolution of the Oracle VirtualBox shared folders name service. To fix these delays, add the following entries to the file \windows\system32\drivers\etc\lmhosts of the Windows guest:

255.255.255.255        VBOXSVR #PRE
255.255.255.255        VBOXSRV #PRE

After doing this change, a reboot of the guest is required.

USB Tablet Coordinates Wrong in Windows 98 Guests

If a Windows 98 VM is configured to use the emulated USB tablet (absolute pointing device), the coordinate translation may be incorrect and the pointer is restricted to the upper left quarter of the guest's screen.

The USB HID (Human Interface Device) drivers in Windows 98 are very old and do not handle tablets in the same way as modern operating systems do. To work around the problem, use the following command:

$ VBoxManage setextradata VM-name "VBoxInternal/USB/HidMouse/0/Config/CoordShift" 0

To restore the default behavior, remove the key or set its value to 1.

Windows Guests are Removed From an Active Directory Domain After Restoring a Snapshot

If a Windows guest is a member of an Active Directory domain and the snapshot feature of Oracle VirtualBox is used, it could be removed from the Active Directory domain after you restore an older snapshot.

This is caused by automatic machine password changes performed by Windows at regular intervals for security purposes. You can disable this feature as shown in this article from Microsoft.

Windows XP Guests Have Slower Transmission Rates

Details: Neither virtio nor Intel PRO/1000 drivers for Windows XP guests support segmentation offloading. Therefore Windows XP guests have slower transmission rates comparing to other guest types. Refer to Microsoft Knowledge base article 842264 for additional information.
Workaround: No workaround exists.

Linux Guests

Repeated Rebuild of Kernel Modules on Reboot of Guest

Details

Some Linux distributions, for example Debian, do not provide tools that VirtualBox Guest Additions uses to verify kernel module signatures. If VirtualBox Guest Additions can't verify a module's signature, they will attempt to rebuild the kernel module on system boot. This can cause boot delays and the need to re-sign modules every time the VM is rebooted.

Workaround

On the VM, create a file /etc/virtualbox-guest-additions.conf, with the content

VBOX_BYPASS_MODULES_SIGNATURE_CHECK=1

This tells VirtualBox Guest Additions not to check signatures on boot, but to load the modules and leave it to the kernel to check module signatures.

If you set the variable to any value other than 1, it has no effect.

Error When Removing a Monitor

Details: VBoxDRMClient can't handle arbitrary guest monitor topologies. Disabling a guest monitor other than the last one invalidates the monitor topology because of limitations in the vmwgfx.ko Linux kernel module.
Workaround: Always remove the last monitor in a the topology. For example, when the guest is configured to have four monitors, avoid disabling the second or third monitor.

Guest Control Not Working

Details: Guest Control File Manager, or other processes to control guest applications from the host using VBoxManage guestcontrol, may be prevented from running by Security-Enhanced Linux (SELinux).
Workaround: In SELinux, if your security policies permit, allow the execve() call for the VBoxService binary only.

Arm Guests

7.1 Saved States and Snapshots Not Usable in 7.2

Details: Saved states and online snapshots created on Arm VMs in Oracle VirtualBox 7.1 are not usable in later releases.
Workaround: Take a snapshot while the VM is powered off. It will then not have a saved state and therefore won't have compatibility issues.

Windows Hosts

CD and DVD Changes Not Recognized

In case you have assigned a physical CD or DVD drive to a guest and the guest does not notice when the medium changes, make sure that the Windows media change notification (MCN) feature is not turned off. This is represented by the following key in the Windows registry:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Cdrom\Autorun

Certain applications may disable this key against Microsoft's advice. If it is set to 0, change it to 1 and reboot your system. Oracle VirtualBox relies on Windows notifying it of media changes.

Issues Accessing Network Shares When VBoxSVC running in Windows Session 0

Due to different Windows sessions having their own set of resources, there might be some issues with accessing network shares created in the interactive user session when at least one of the Oracle VirtualBox processes are running in session 0.

For accessing network shares within session 0, a possible workaround is to establish permanent access to the share and then restart the host.

Sluggish Response When Using Microsoft RDP Client

If connecting to a Virtual Machine using the Microsoft RDP client, called a Remote Desktop Connection, there can be large delays between input such as moving the mouse over a menu and output. This is because this RDP client collects input for a certain time before sending it to the RDP server.

The interval can be decreased by setting a Windows registry key to smaller values than the default of 100. The key does not exist initially and must be of type DWORD. The unit for its values is milliseconds. Values around 20 are suitable for low-bandwidth connections between the RDP client and server. Values around 4 can be used for a gigabit Ethernet connection. Generally values below 10 achieve a performance that is very close to that of the local input devices and screen of the host on which the Virtual Machine is running.

Depending whether the setting should be changed for an individual user or for the system, set either of the following.

HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Min Send Interval

HKEY_LOCAL_MACHINE\Software\Microsoft\Terminal Server Client\Min Send Interval

Running an iSCSI Initiator and Target on a Single System

Deadlocks can occur on a Windows host when attempting to access an iSCSI target running in a guest virtual machine with an iSCSI initiator, such as a Microsoft iSCSI Initiator, that is running on the host. This is caused by a flaw in the Windows cache manager component, and causes sluggish host system response for several minutes, followed by a "Delayed Write Failed" error message in the system tray or in a separate message window. The guest is blocked during that period and may show error messages or become unstable.

Setting the VBOX_DISABLE_HOST_DISK_CACHE environment variable to 1 enables a workaround for this problem until Microsoft addresses the issue. For example, open a command prompt window and start Oracle VirtualBox like this:

set VBOX_DISABLE_HOST_DISK_CACHE=1
VirtualBox

While this will decrease guest disk performance, especially writes, it does not affect the performance of other applications running on the host.

Bridged Networking Adapters Missing

If no bridged adapters show up in the Networking section of the VM settings, this typically means that the bridged networking driver was not installed properly on your host. This could be due to the following reasons:

The maximum allowed filter count was reached on the host. In this case, the MSI log would mention the 0x8004a029 error code returned on NetFlt network component install, as follows:
```
VBoxNetCfgWinInstallComponent: Install failed, hr (0x8004a029)
```
You can try to increase the maximum filter count in the Windows registry using the following key:
```
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Network\MaxNumFilters
```
The maximum number allowed is 14. After a reboot, try to reinstall Oracle VirtualBox.
The INF cache is corrupt. In this case, the install log at %windir%\inf\setupapi.dev.log would typically mention the failure to find a suitable driver package for either the sun_VBoxNetFlt or sun_VBoxNetFltmp components. The solution then is to uninstall Oracle VirtualBox, remove the INF cache (%windir%\inf\INFCACHE.1), reboot and try to reinstall Oracle VirtualBox.

Host-Only Networking Adapters Cannot be Created

If a host-only adapter cannot be created, either with the Oracle VirtualBox Manager or the VBoxManage command, then the INF cache is probably corrupt. In this case, the install log at %windir%\inf\setupapi.dev.log would typically mention the failure to find a suitable driver package for the sun_VBoxNetAdp component. Again, as with the bridged networking problem described above, the solution is to uninstall Oracle VirtualBox, remove the INF cache (%windir%\inf\INFCACHE.1), reboot and try to reinstall Oracle VirtualBox.

Poor performance when using Oracle VirtualBox and Hyper-V on the same host

Details

Oracle VirtualBox and Hyper-V can conflict with each other when run on the same host.

Always disable Hyper-V when running VirtualBox.

Workaround

Turn off the Windows features Hyper-V Platform, Virtual Machine Platform and Windows Hypervisor Platform, and then reboot the host.

On newer Windows versions, the device security features Core Isolation and Memory Integrity use Hyper-V, even if you have turned it off.

Use the Microsoft Device Guard and Credential Guard hardware readiness tools to turn off more features that use Hyper-V. For example, to disable AutoReboot, use the following command:

.\DG_Readiness_Tool_vX.X.ps1 -Disable -AutoReboot

Note:

Disabling Device Guard and Credential Guard features impacts on the overall security of the host. Take this into account before deciding to disable them.

Linux Hosts

Repeated Rebuild of Kernel Modules on Reboot of Host

Details

Some Linux distributions, for example Debian, do not provide tools that VirtualBox uses to verify kernel module signatures. If VirtualBox can't verify a module's signature, it will attempt to rebuild the kernel module on system boot. This can cause boot delays and the need to re-sign modules every time the machine is rebooted.

Workaround

On the host machine, edit the file /etc/vbox/vbox.cfg, and add the line

VBOX_BYPASS_MODULES_SIGNATURE_CHECK=1

This tells VirtualBox not to check signatures on boot, but to load the modules and leave it to the kernel to check module signatures.

If you set the variable to any value other than 1, it has no effect.

Linux Kernel Module Refuses to Load

If the Oracle VirtualBox kernel module, vboxdrv, refuses to load you may see an Error inserting vboxdrv: Invalid argument message. As root, check the output of the dmesg command to find out why the load failed. Most probably the kernel disagrees with the version of gcc used to compile the module. Make sure that you use the same compiler that was used to build the kernel.

Linux Host CD/DVD or Floppy Disk Drive Not Found

If you have configured a virtual machine to use the host's CD or DVD drive or floppy disk drive, but this does not appear to work, make sure that the current user has permission to access the corresponding Linux device file. For example, for a CD or DVD drive this may be /dev/hdc, /dev/scd0, /dev/cdrom or similar. On most distributions, the user must be added to a corresponding group, usually called cdrom or cdrw or floppy.

On supported Linux distributions, Oracle VirtualBox uses udev to locate hardware such as CD/DVD drives and floppy disk drives.

Strange Guest IDE Error Messages When Writing to CD or DVD

If the experimental CD or DVD writer support is enabled with an incorrect host or guest configuration, it is possible that any attempt to access the CD or DVD writer fails and simply results in guest kernel error messages for Linux guests or application error messages for Windows guests. Oracle VirtualBox performs the usual consistency checks when a VM is powered up. In particular, it aborts with an error message if the device for the CD or DVD writer is not writable by the user starting the VM. But Oracle VirtualBox cannot detect all misconfigurations. The necessary host and guest OS configuration is not specific for Oracle VirtualBox, but a few frequent problems are listed here which occurred in connection with Oracle VirtualBox.

Special care must be taken to use the correct device. The configured host CD or DVD device file name, in most cases /dev/cdrom, must point to the device that allows writing to the CD or DVD unit. For CD or DVD writer units connected to a SCSI controller or to a IDE controller that interfaces to the Linux SCSI subsystem, common for some SATA controllers, this must refer to the SCSI device node, such as /dev/scd0. Even for IDE CD or DVD writer units this must refer to the appropriate SCSI CD-ROM device node, such as /dev/scd0, if the ide-scsi kernel module is loaded. This module is required for CD or DVD writer support with some early 2.6 kernels. Many Linux distributions load this module whenever a CD or DVD writer is detected in the system, even if the kernel would support CD or DVD writers without the module. Oracle VirtualBox supports the use of IDE device files, such as /dev/hdc, provided the kernel supports this and the ide-scsi module is not loaded.

Similar rules, except that within the guest the CD or DVD writer is always an IDE device, apply to the guest configuration. Since this setup is very common, it is likely that the default configuration of the guest works as expected.

VBoxSVC IPC Issues

On Linux, Oracle VirtualBox makes use of a custom version of Mozilla XPCOM (cross platform component object model) for interprocess and intraprocess communication (IPC). The process VBoxSVC serves as a communication hub between different Oracle VirtualBox processes and maintains the global configuration, such as the XML database. When starting an Oracle VirtualBox component, the processes VBoxSVC and VBoxXPCOMIPCD are started automatically. They are only accessible from the user account they are running under. VBoxSVC owns the Oracle VirtualBox configuration database which normally resides in ~/.config/VirtualBox, or the appropriate configuration directory for your operating system. While it is running, the configuration files are locked. Communication between the various Oracle VirtualBox components and VBoxSVC is performed through a local domain socket residing in /tmp/.vbox-username-ipc. In case there are communication problems, such as an Oracle VirtualBox application cannot communicate with VBoxSVC, terminate the daemons and remove the local domain socket directory.

USB Not Working

If USB is not working on your Linux host, make sure that the current user is a member of the vboxusers group. Please remember that group membership does not take effect immediately but rather at the next login. If available, the newgrp command may avoid the need for a logout and login.

PAX/grsec Kernels

Linux kernels including the grsec patch, see http://www.grsecurity.net/, and derivates have to disable PAX_MPROTECT for the VBox binaries to be able to start a VM. The reason is that Oracle VirtualBox has to create executable code on anonymous memory.

Linux Kernel vmalloc Pool Exhausted

When running a large number of VMs with a lot of RAM on a Linux system, say 20 VMs with 1 GB of RAM each, additional VMs might fail to start with a kernel error saying that the vmalloc pool is exhausted and should be extended. The error message also tells you to specify vmalloc=256MB in your kernel parameter list. If adding this parameter to your GRUB or LILO configuration makes the kernel fail to boot, with an error message such as failed to mount the root partition, then you have probably run into a memory conflict of your kernel and initial RAM disk. This can be solved by adding the following parameter to your GRUB configuration:

uppermem 524288

Oracle Solaris Hosts

Cannot Start VM, Not Enough Contiguous Memory

Details: The ZFS file system's Adaptive Replacement Cache (ARC) uses a portion of the system's RAM to store frequently accessed data to significantly speed up accessing and reading file system data. The memory used by the ZFS ARC can become fragmented (for example, when cached file system data is deleted and the kernel has yet to recover the memory buffers). This can prevent applications such as VirtualBox from starting.
Workaround: Solaris 11.1 SRU 20.5 and later includes a kernel tunable parameter,user_reserve_hint_pct, which can be set to allow system administrators to specify a reserve of free memory for applications by restricting growth of the ZFS ARC cache. See the Oracle Solaris 11.4 TunableParameters Reference Manual(https://docs.oracle.com/en/operating-systems/solaris/oracle-solaris/11.4/tuning/user_reserve_hint_pct-zfs-parameter.html) and Document 1663862.1, Memory Management Between ZFS and Applications in Oracle Solaris 11.x, in My Oracle Support (MOS) for further information and guidance on tuning this parameter.

macOS Hosts

Internal Network Not Working

Details: Internal networking on macOS hosts doesn't work between VMs run by different users.
Workaround: We're working on a resolution.

macOS Host Limitations

Details

On macOS hosts, the following features are not yet implemented:

Numlock emulation
CPU frequency metric
Memory ballooning

macOS/Arm 64 (Apple silicon) hosts have the following limitations:

x86-based guest operating systems will not run.
Arm(AArch64) guests only. Arm 32 is not supported at present.
Arm hosts have limitations with sound, storage, graphics, guest additions and unattended installation.

Workaround

We're working on a resolution.