Re: [Qemu-devel] [PATCH 3/5] vfio/quirks: ioeventfd quirk acceleration

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 28, 2018 at 01:45:54PM -0700, Alex Williamson wrote:
> 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.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

I don't know when will there be non-dynamic vfio-ioeventfds, but it
looks fine at least to me even if all of them are dynamic now:

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu