Re: [Qemu-devel] [PATCH v2 2/2] kvm: migrate vPMU state

[Gleb Natapov <gleb@redhat.com>]

On Sun, Jul 28, 2013 at 03:51:25PM +0200, Paolo Bonzini wrote:
> Il 28/07/2013 14:57, Gleb Natapov ha scritto:
> >> @@ -1114,6 +1135,33 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
> >>              kvm_msr_entry_set(&msrs[n++], MSR_KVM_STEAL_TIME,
> >>                                env->steal_time_msr);
> >>          }
> >> +        if (has_msr_architectural_pmu) {
> >> +            /* Stop the counter.  */
> >> +            kvm_msr_entry_set(&msrs[n++], MSR_CORE_PERF_FIXED_CTR_CTRL, 0);
> >> +            kvm_msr_entry_set(&msrs[n++], MSR_CORE_PERF_GLOBAL_CTRL, 0);
> >> +
> > Why is this needed?
> 
> In v1 it was in the commit message.  I'll fix it up before applying:
> 
> > Second, to avoid any possible side effects during the setting of MSRs
> > I stop the PMU while setting the counters and event selector MSRs.
> > Stopping the PMU snapshots the counters and ensures that no strange
> > races can happen if the counters were saved close to their overflow
> > value.
> 
Since vcpu is not running counters should not count anyway.


--
			Gleb.