Both standard (MSI) and extended (MSI-X) message-signaled interrupts are implemented as in-band messages. A message-signaled interrupt is posted as a write with an address and value that are specified by the software.
Conventional PCI specifications include optional support for Message Signaled Interrupts (MSI). An MSI is an in-band message that is implemented as a posted write. The address and the data for the MSI are specified by software and are specific to the host bridge. Because the messages are in-band, the receipt of the message can be used to “push” data that is associated with the interrupt. By definition, MSI interrupts are unshared. Each MSI message that is assigned to a device is guaranteed to be a unique message in the system. PCI functions can request 1, 2, 4, 8, 16, or 32 MSI messages. Note that the system software can allocate fewer MSI messages to a function than the function requested. The host bridge can be limited in the number of unique MSI messages that are allocated for devices.
A maximum of 2048 MSI-X interrupt vectors are supported per device.
Address and data entries are unique per interrupt vector.
MSI-X supports per function masking and per vector masking.
With MSI-X interrupts, an unallocated interrupt vector of a device can use a previously added or initialized MSI-X interrupt vector to share the same vector address, vector data, interrupt handler, and handler arguments. Use the ddi_intr_dup_handler(9F) function to alias the resources provided by the Solaris OS to the unallocated interrupt vectors on an associated device. For example, if 2 MSI-X interrupts are allocated to a driver and 32 interrupts are supported on the device, then the driver can use ddi_intr_dup_handler() to alias the 2 interrupts it received to the 30 additional interrupts on the device.
A duplicated interrupt is disabled initially. Use ddi_intr_enable() to enable the duplicated interrupt. You cannot remove the original MSI-X interrupt handler until all duplicated interrupt handlers that are associated with this original interrupt handler are removed. To remove a duplicated interrupt handler, first call ddi_intr_disable(9F), and then call ddi_intr_free(9F). When all duplicated interrupt handlers that are associated with this original interrupt handler are removed, then you can use ddi_intr_remove_handler(9F) to remove the original MSI-X interrupt handler. See the ddi_intr_dup_handler(9F) man page for examples.