Re: perf uncore & lkvm woes

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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.

> 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.

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