Re: KVM: arm64: pmu: Reset sample period on overflow handling

[Marc Zyngier <maz@kernel.org>]

On Wed, 16 Jun 2021 11:52:42 +0100,
Aman Priyadarshi <apeureka@amazon.de> wrote:
> 
> On Wed, 2021-06-16 at 11:31 +0100, Marc Zyngier wrote:
> > 
> > Hi Aman,
> > 
> > On Wed, 16 Jun 2021 10:17:28 +0100,
> > Aman Priyadarshi <apeureka@amazon.de> wrote:
> > > 
> > > Hi Marc,
> > > 
> > > On Tue, 2021-06-15 at 18:05 +0100, Marc Zyngier wrote:
> > > > 
> > > > Can you reproduce the issue with vanilla guest kernels? It'd be
> > > > interesting to understand what makes it work on the guest side. Can
> > > > you please bisect it?
> > > > 
> > > 
> > > yes, I was able to narrow it down to the commit 0cbb058be904 ("arm64:
> > > perf:
> > > Disable PMU while processing counter overflows"), which fixes the
> > > problem
> > > on the guest side.
> > 
> > Which is 3cce50dfec4a5b0414c974190940f47dd32c6dee in mainline. This
> > doesn't seem to have ever been backported before 4.18. So I don't know
> > why your 4.15 kernel was correctly behaving, but it could be that the
> > distro had randomly picked up the correct patch!
> 
> Yes. https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1836117

Right. I guess that's their "value add".

> > You may want to backport it to 4.14.y and let Greg know about that.
> > 
> 
> Ack.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.
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