Re: perf uncore & lkvm woes

[Avi Kivity <avi@redhat.com>]

On 08/21/2012 01:35 PM, Peter Zijlstra wrote:
> On Tue, 2012-08-21 at 11:34 +0300, Avi Kivity wrote:
>> On 08/21/2012 10:11 AM, Peter Zijlstra wrote:
>> > On Sun, 2012-08-19 at 12:55 +0300, Avi Kivity wrote:
>> >> > I think Avi prefers the method where KVM 'fakes' the MSRs and we have to
>> >> > detect if the MSRs actually work or not.
>> >> 
>> >> s/we have/we don't have/. 
>> > 
>> > So for the 'normal' PMU we actually do check to see if the MSRs are
>> > being faked and bail if they are.
>> 
>> That was because earlier versions of kvm did not virtualize the pmu.
>> 
>> The approaches are not mutually exclusive.  We can check in the guest,
>> and fake it in the host.
> 
> This is actually what I proposed.

Ah, I misunderstood you.

> 
>> The problem with faking it in the host is if someone actually relies on
>> the pmu for something, not just instrumentation.  We do that for the
>> watchdog, but I don't see it happening with the uncore pmu.
> 
> Agreed, although from a usability POV its nicer to refuse the
> device/events than to pretend it works while it doesn't.

If/when we virtualize this pmu we can expose a flag that says "the pmu
actually works even though this is a virtual machine".

> Anyway, for now I've taken Zheng Yan's cpu_has_hypervisor patch, we can
> always revisit this.

Yup.

-- 
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