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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/20/2016 10:11 AM, Andrew Cooper wrote:
> 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.

I am actually not the maintainer, I just break this code more often than
others.

But yes, having a test case would make it much easier to understand what
and why is not working.

Would something like

    wrmsr(PERFCTR,0);
    wrmsr(EVNTSEL, XXX); //enable counter
    // do something simple, with branches
    wrmsr(EVTSEL,YYY); // disable counter
   
demonstrate the problem? (I assume we are talking about HVM guest)

-boris

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