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

[Paolo Bonzini <pbonzini@redhat.com>]

> 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.

Yes, you're right.

> 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)

Why do you need "if (msr_info->host_initiated)"?  I could not find any
hint in the manual that the overflow counter will still use the value
of the counter that was programmed first.

If we need to do it always, I agree it's better to modify the QEMU
patch and not disable/enable the counters.  But if we need to restrict
it to host-initiated writes, I would rather have the QEMU patch as I
posted it.  So far we always had less side-effects from host_initiated,
not more, and I think it's a good rule of thumb.

Paolo