linux-kernel - Re: [PATCH AUTOSEL 5.12 42/43] virtio: fix up virtio_disable

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Message-ID: <YPOGxjBcq6f4l+0v@sashalap>
Date:   Sat, 17 Jul 2021 21:41:26 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH AUTOSEL 5.12 42/43] virtio: fix up virtio_disable_cb

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@...hat.com>
>>
>> [ 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@...hat.com>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>
>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