The bandwidth value provided in a fibre channel SR-IOV callback is specified in a percentile format rather than an absolute bandwidth percentage. Specifying a percentile provides flexibility for the PF driver and the HBA firmware to tune the hardware to meet the quality of service (QoS) that is close to the specified percentile. The devices are expected to provide QoS as accurately as possible.
The bandwidth percentile applies only to the bandwidth that is available to the set of virtual functions associated with a PF. Bandwidth used by the PF itself must be reserved through some other means.
The bandwidth configuration is specified as the minimum cap that must be reserved for a particular VF. The PF driver can choose to allocate more available bandwidth to a VF if it does not cause other virtual functions to fall below their specified minimum. The following guidelines describes how a PF driver interprets the bandwidth parameters to a callback:
All virtual functions that have no explicit bandwidth percentile set or a bandwidth percentile of zero should get a fair share allocation of the bandwidth that is not reserved for any other VF.
For example, if 60% of bandwidth is allocated for one or more virtual functions, then the rest (40%) of the bandwidth will be used for fair share allocation between all virtual functions that have no bandwidth percentile specified.
The percentile of the available bandwidth for virtual functions associated with this PF.
The PF driver must ensure that if there are any virtual functions that have no bandwidth percentile specified, some bandwidth is available for those virtual functions. A situation in which a VF has no available bandwidth should be prevented.
Bandwidth allocations are not intended to be a hard requirement that must be met. A best effort attempt by the PF driver and HBA firmware is all that is expected.