From: David Matlack <dmatlack@google.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: pbonzini@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kvm: x86: add trace event for pvclock updates
Date: Wed, 12 Nov 2014 10:00:50 -0800 [thread overview]
Message-ID: <20141112180050.GA22530@google.com> (raw)
In-Reply-To: <20141111011850.GA12749@amt.cnet>
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@google.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.
>
>
prev parent reply other threads:[~2014-11-12 18:00 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-05 19:46 [PATCH] kvm: x86: add trace event for pvclock updates David Matlack
2014-11-06 10:53 ` Paolo Bonzini
2014-11-11 1:18 ` Marcelo Tosatti
2014-11-12 18:00 ` David Matlack [this message]
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