Messages in this thread |  | | Date | Wed, 12 Nov 2014 10:00:50 -0800 | From | David Matlack <> | Subject | Re: [PATCH] kvm: x86: add trace event for pvclock updates |
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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.
> >
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