Re: [Xen-devel] VPMU interrupt unreliability

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

>> One thing I noticed is that the workaround doesn't appear to be
>> complete: it is only checking PMC0 status and not other counters (fixed
>> or architectural). Of course, without knowing what the actual problem
>> was it's hard to say whether this was intentional.
> handle_pmc_quirk appears to loop through all the counters ...

Right, I didn't notice that it is shifting MSR_CORE_PERF_GLOBAL_STATUS
value one by one and so it is looking at all bits.

>
>>> 2. Intercepting MSR loads for counters that have the workaround
>>> applied and giving the guest the correct counter value.
>>
>> We'd have to keep track of whether the counter has been reset (by the
>> quirk) since the last MSR write.
> Yes.
>
>>> 3. Or perhaps even changing the workaround to disable the PMI on that
>>> counter until the guest acks via GLOBAL_OVF_CTRL, assuming that works
>>> on the relevant hardware.
>> MSR_CORE_PERF_GLOBAL_OVF_CTRL is written immediately after the quirk
>> runs (in core2_vpmu_do_interrupt()) so we already do this, don't we?
> I'm suggesting waiting until the *guest* writes to the (virtualized)
> GLOBAL_OVF_CTRL.

Wouldn't it be better to wait until the counter is reloaded?


-boris

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