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Date: Wed, 3 Jan 2024 15:27:29 -0800
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Like Xu <like.xu@linux.intel.com>, Andi Kleen <ak@linux.intel.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	Luwei Kang <luwei.kang@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, Breno Leitao <leitao@debian.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Ingo Molnar <mingo@redhat.com>
Subject: Re: [BUG] Guest OSes die simultaneously (bisected)
Message-ID: <88f49775-2b56-48cc-81b8-651a940b7d6b@paulmck-laptop>
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References: <3d8f5987-e09c-4dd2-a9c0-8ba22c9e948a@paulmck-laptop>
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On Wed, Jan 03, 2024 at 02:22:23PM -0800, Paul E. McKenney wrote:
> Hello!
> 
> Since some time between v5.19 and v6.4, long-running rcutorture tests
> would (rarely but intolerably often) have all guests on a given host die
> simultaneously with something like an instruction fault or a segmentation
> violation.
> 
> Each bisection step required 20 hosts running 10 hours each, and
> this eventually fingered commit c59a1f106f5c ("KVM: x86/pmu: Add
> IA32_PEBS_ENABLE MSR emulation for extended PEBS").  Although this commit
> is certainly messing with things that could possibly cause all manner
> of mischief, I don't immediately see a smoking gun.  Except that the
> commit prior to this one is rock solid.
> 
> Just to make things a bit more exciting, bisection in mainline proved
> to be problematic due to bugs of various kinds that hid this one.  I was
> therefore forced to bisect among the commits backported to the internal
> v5.19-based kernel, which fingered the backported version of the patch
> called out above.

Ah, and so why do I believe that this is a problem in mainline rather
than just (say) a backporting mistake?

Because this issue was first located in v6.4, which already has this
commit included.

							Thanx, Paul

> Please note that this is not (yet) an emergency.  I will just continue
> to run rcutorture on v5.19-based hypervisors in the meantime.
> 
> Any suggestions for debugging or fixing?
> 
> 							Thanx, Paul