Re: [Xen-devel] [PATCH v3 2/2] x86/Intel: virtualize support for cpuid faulting

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/10/16 14:55, Kyle Huey wrote:
>
>>> That said, rr currently does not work in Xen guests due to some PMU
>>> issues that we haven't tracked down yet.
>> Is this RR trying to use vPMU and it not functioning, or not
>> specifically trying to use PMU facilities and getting stuck anyway?
> The latter.  rr relies on the values returned by the PMU (the retired
> conditional branches counter in particular) being exactly the same
> during the recording and replay phases.  This is true when running on
> bare metal, and when running inside a KVM guest, but when running in a
> Xen HVM guest we see values that are off by a branch or two on a small
> fraction of our tests.  Since it works in KVM I suspect this is some
> sort of issue with how Xen multiplexes the real PMU and events are
> "leaking" between guests (or perhaps from Xen itself, though I don't
> think the Xen kernel executes any ring 3 code).  Even if that's
> correct we're a long way from tracking it down and patching it though.

Hmm.  That is unfortunate, and does point towards a bug in Xen.  Are
these tests which notice the problem easy to run?

Boris (CC'd) is the maintainer of that code.  It has undergone quite a
few changes recently.

~Andrew

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