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vfio/quirks: ioeventfd quirk acceleration
The NVIDIA BAR0 quirks virtualize the PCI config space mirrors found in device MMIO space. Normally PCI config space is considered a slow path and further optimization is unnecessary, however NVIDIA uses a register here to enable the MSI interrupt to re-trigger. Exiting to QEMU for this MSI-ACK handling can therefore rate limit our interrupt handling. Fortunately the MSI-ACK write is easily detected since the quirk MemoryRegion otherwise has very few accesses, so simply looking for consecutive writes with the same data is sufficient, in this case 10 consecutive writes with the same data and size is arbitrarily chosen. We configure the KVM ioeventfd with data match, so there's no risk of triggering for the wrong data or size, but we do risk that pathological driver behavior might consume all of QEMU's file descriptors, so we cap ourselves to 10 ioeventfds for this purpose. In support of the above, generic ioeventfd infrastructure is added for vfio quirks. This automatically initializes an ioeventfd list per quirk, disables and frees ioeventfds on exit, and allows ioeventfds marked as dynamic to be dropped on device reset. The rationale for this latter feature is that useful ioeventfds may depend on specific driver behavior and since we necessarily place a cap on our use of ioeventfds, a machine reset is a reasonable point at which to assume a new driver and re-profile. Reviewed-by: Peter Xu <peterx@redhat.com> Reviewed-by: Eric Auger <eric.auger@redhat.com> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
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