Re: [PATCH v2 26/54] KVM: x86/pmu: Avoid legacy vPMU code when accessing global_ctrl in passthrough vPMU

Mingwei Zhang
2024-05-30T05:28:50Z

On Thu, May 09, 2024, Mi, Dapeng wrote:
>
> On 5/9/2024 9:29 AM, Chen, Zide wrote:
> >
> > On 5/8/2024 5:43 PM, Mi, Dapeng wrote:
> >> On 5/9/2024 5:48 AM, Chen, Zide wrote:
> >>> On 5/5/2024 10:29 PM, Mingwei Zhang wrote:
> >>>> Avoid calling into legacy/emulated vPMU logic such as reprogram_counters()
> >>>> when passthrough vPMU is enabled. Note that even when passthrough vPMU is
> >>>> enabled, global_ctrl may still be intercepted if guest VM only sees a
> >>>> subset of the counters.
> >>>>
> >>>> Suggested-by: Xiong Zhang <xiong.y.zhang@linux.intel.com>
> >>>> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> >>>> ---
> >>>> arch/x86/kvm/pmu.c | 3 ++-
> >>>> 1 file changed, 2 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> >>>> index bd94f2d67f5c..e9047051489e 100644
> >>>> --- a/arch/x86/kvm/pmu.c
> >>>> +++ b/arch/x86/kvm/pmu.c
> >>>> @@ -713,7 +713,8 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >>>> if (pmu->global_ctrl != data) {
> >>>> diff = pmu->global_ctrl ^ data;
> >>>> pmu->global_ctrl = data;
> >>>> - reprogram_counters(pmu, diff);
> >>>> + if (!is_passthrough_pmu_enabled(vcpu))
> >>>> + reprogram_counters(pmu, diff);
> >>> Since in [PATCH 44/54], reprogram_counters() is effectively skipped in
> >>> the passthrough case, is this patch still needed?
> >> Zide, reprogram_counters() and reprogram_counter() are two different
> >> helpers. Both they need to be skipped in passthrough mode.
> > Yes, but this is talking about reprogram_counters() only. passthrough
> > mode is being checked inside and outside the function call, which is
> > redundant.
> Oh, yes. I don't need this patch then.

Right. I am thinking about dropping [PATCH 44/54], since that one
contains some redundant checking. I will see how should this be fixed in
next version. Thanks for pointing it out though.

-Mingwei