Re: [PATCH] virtio: don't enable notifications during polling

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Dec 11, 2019 at 10:58:43AM -0500, Michael S. Tsirkin wrote:
> On Mon, Dec 09, 2019 at 09:09:57PM +0000, Stefan Hajnoczi wrote:
> > Virtqueue notifications are not necessary during polling, so we disable
> > them.  This allows the guest driver to avoid MMIO vmexits.
> > Unfortunately the virtio-blk and virtio-scsi handler functions re-enable
> > notifications, defeating this optimization.
> > 
> > Fix virtio-blk and virtio-scsi emulation so they leave notifications
> > disabled.  The key thing to remember for correctness is that polling
> > always checks one last time after ending its loop, therefore it's safe
> > to lose the race when re-enabling notifications at the end of polling.
> > 
> > There is a measurable performance improvement of 5-10% with the null-co
> > block driver.  Real-life storage configurations will see a smaller
> > improvement because the MMIO vmexit overhead contributes less to
> > latency.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> 
> Thanks! I'll queue it for merge after the release. If possible please ping me
> after the release to help make sure it didn't get dropped.

Great, thanks!

Stefan

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