Re: [PATCH AUTOSEL 5.12 42/43] virtio: fix up virtio_disable_cb

[Sasha Levin]

On Sun, Jul 11, 2021 at 12:23:59AM -0400, Michael S. Tsirkin wrote:
> On Sat, Jul 10, 2021 at 07:49:14PM -0400, Sasha Levin wrote:
> > From: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> >
> > [ Upstream commit 8d622d21d24803408b256d96463eac4574dcf067 ]
> >
> > virtio_disable_cb is currently a nop for split ring with event index.
> > This is because it used to be always called from a callback when we know
> > device won't trigger more events until we update the index.  However,
> > now that we run with interrupts enabled a lot we also poll without a
> > callback so that is different: disabling callbacks will help reduce the
> > number of spurious interrupts.
> > Further, if using event index with a packed ring, and if being called
> > from a callback, we actually do disable interrupts which is unnecessary.
> >
> > Fix both issues by tracking whenever we get a callback. If that is
> > the case disabling interrupts with event index can be a nop.
> > If not the case disable interrupts. Note: with a split ring
> > there's no explicit "no interrupts" value. For now we write
> > a fixed value so our chance of triggering an interupt
> > is 1/ring size. It's probably better to write something
> > related to the last used index there to reduce the chance
> > even further. For now I'm keeping it simple.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
>
> I am not sure we want this in stable yet. It should in theory
> fix the reported interrupt storms but the reporter is on vacation.

Sure, I'll drop it for now. Let me know when you want it re-added.
Thanks!

--
Thanks,
Sasha