linux-kernel - Re: [PATCH] kvm: x86: add trace event for pvclock updates

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Date:	Wed, 12 Nov 2014 10:00:50 -0800
From:	David Matlack <dmatlack@...gle.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	pbonzini@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: add trace event for pvclock updates

On 11/10 11:18 PM, Marcelo Tosatti wrote:
> On Wed, Nov 05, 2014 at 11:46:42AM -0800, David Matlack wrote:
> > The new trace event records:
> >   * the id of vcpu being updated
> >   * the pvclock_vcpu_time_info struct being written to guest memory
> > 
> > This is useful for debugging pvclock bugs, such as the bug fixed by
> > "[PATCH] kvm: x86: Fix kvm clock versioning.".
> > 
> > Signed-off-by: David Matlack <dmatlack@...gle.com>
> 
> So you actually hit that bug in practice? Can you describe the
> scenario?

We noticed guests running stress workloads would occasionally get stuck 
on the far side of a save/restore. Inspecting the guest state revealed
arch/x86/kernel/pvclock.c:last_value was stuck at a value like
8020566108469899263, despite TSC and pvclock looking sane.

Since these guests ran without PVCLOCK_TSC_STABLE_BIT set in their
CPUID, they were stuck with this large time value until real time
caught up (in about 250 years :).

We've been unable to reproduce the bug with "kvm: x86: Fix kvm clock
versioning." so we didn't invest in catching the overflow in the act,
but a likely explanation is the guest gets save/restore-ed while
computing the pvclock delta:

        u64 delta = __native_read_tsc() - src->tsc_timestamp;

causing the subtraction to underflow and delta to be huge.

> 
> 
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