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

[Gleb Natapov]

On Sun, Jul 28, 2013 at 04:07:37PM +0200, Paolo Bonzini wrote:
> Il 28/07/2013 15:54, Gleb Natapov ha scritto:
> > 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.
> 
> Does the perf event distinguish KVM_RUN from any other activity in the
> vCPU thread (in which this code runs)?  It seemed unsafe to me to change
> the overflow status and the performance counter value while the counter
> could be running, since the counter value could affect the overflow
> status.  Maybe I was being paranoid?
> 
KVM disabled HW counters when outside of a guest mode (otherwise result
will be useless), so I do not see how the problem you describe can
happen. On the other hand MPU emulation assumes that counter have to be disabled
while MSR_IA32_PERFCTR0 is written since write to MSR_IA32_PERFCTR0 does
not reprogram perf evens, so we need either disable/enable counters to
write MSR_IA32_PERFCTR0 or have this patch in the kernel:


diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 5c4f631..bf14e42 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -412,6 +412,8 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data 
*msr_info)
                        if (!msr_info->host_initiated)
                                data = (s64)(s32)data;
                        pmc->counter += data - read_pmc(pmc);
+                       if (msr_info->host_initiated)
+                               reprogram_gp_counter(pmc, pmc->eventsel);
                        return 0;
                } else if ((pmc = get_gp_pmc(pmu, index, MSR_P6_EVNTSEL0))) {
                        if (data == pmc->eventsel)

--
                        Gleb.