linux-kernel - Re: [PATCH] KVM: x86/svm/pmu: Set PerfMonV2 global control bits correctly

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Message-ID: <ZeYsQLs-_urXpdml@google.com>
Date: Mon, 4 Mar 2024 20:17:04 +0000
From: Mingwei Zhang <mizhang@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Dapeng Mi <dapeng1.mi@...ux.intel.com>,
	Sandipan Das <sandipan.das@....com>,
	Like Xu <like.xu.linux@...il.com>, pbonzini@...hat.com,
	jmattson@...gle.com, ravi.bangoria@....com, nikunj.dadhania@....com,
	santosh.shukla@....com, manali.shukla@....com, babu.moger@....com,
	kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/svm/pmu: Set PerfMonV2 global control bits
 correctly

On Mon, Mar 04, 2024, Sean Christopherson wrote:
> On Mon, Mar 04, 2024, Dapeng Mi wrote:
> > 
> > On 3/1/2024 5:00 PM, Sandipan Das wrote:
> > > On 3/1/2024 2:07 PM, Like Xu wrote:
> > > > On 1/3/2024 3:50 pm, Sandipan Das wrote:
> > > > > With PerfMonV2, a performance monitoring counter will start operating
> > > > > only when both the PERF_CTLx enable bit as well as the corresponding
> > > > > PerfCntrGlobalCtl enable bit are set.
> > > > > 
> > > > > When the PerfMonV2 CPUID feature bit (leaf 0x80000022 EAX bit 0) is set
> > > > > for a guest but the guest kernel does not support PerfMonV2 (such as
> > > > > kernels older than v5.19), the guest counters do not count since the
> > > > > PerfCntrGlobalCtl MSR is initialized to zero and the guest kernel never
> > > > > writes to it.
> > > > If the vcpu has the PerfMonV2 feature, it should not work the way legacy
> > > > PMU does. Users need to use the new driver to operate the new hardware,
> > > > don't they ? One practical approach is that the hypervisor should not set
> > > > the PerfMonV2 bit for this unpatched 'v5.19' guest.
> > > > 
> > > My understanding is that the legacy method of managing the counters should
> > > still work because the enable bits in PerfCntrGlobalCtl are expected to be
> > > set. The AMD PPR does mention that the PerfCntrEn bitfield of PerfCntrGlobalCtl
> > > is set to 0x3f after a system reset. That way, the guest kernel can use either
> > 
> > If so, please add the PPR description here as comments.
> 
> Or even better, make that architectural behavior that's documented in the APM.
> 
> > > > > ---
> > > > >    arch/x86/kvm/svm/pmu.c | 1 +
> > > > >    1 file changed, 1 insertion(+)
> > > > > 
> > > > > diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> > > > > index b6a7ad4d6914..14709c564d6a 100644
> > > > > --- a/arch/x86/kvm/svm/pmu.c
> > > > > +++ b/arch/x86/kvm/svm/pmu.c
> > > > > @@ -205,6 +205,7 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
> > > > >        if (pmu->version > 1) {
> > > > >            pmu->global_ctrl_mask = ~((1ull << pmu->nr_arch_gp_counters) - 1);
> > > > >            pmu->global_status_mask = pmu->global_ctrl_mask;
> > > > > +        pmu->global_ctrl = ~pmu->global_ctrl_mask;
> > 
> > It seems to be more easily understand to calculate global_ctrl firstly and
> > then derive the globol_ctrl_mask (negative logic).
> 
> Hrm, I'm torn.  On one hand, awful name aside (global_ctrl_mask should really be
> something like global_ctrl_rsvd_bits), the computation of the reserved bits should
> come from the capabilities of the PMU, not from the RESET value.
> 
+1

> On the other hand, setting _all_ non-reserved bits will likely do the wrong thing
> if AMD ever adds bits in PerfCntGlobalCtl that aren't tied to general purpose
> counters.  But, that's a future theoretical problem, so I'm inclined to vote for
> Sandipan's approach.
> 

right. I am ok with either approach.

Thanks.
-Mingwei
> > diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> > index e886300f0f97..7ac9b080aba6 100644
> > --- a/arch/x86/kvm/svm/pmu.c
> > +++ b/arch/x86/kvm/svm/pmu.c
> > @@ -199,7 +199,8 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
> > kvm_pmu_cap.num_counters_gp);
> > 
> >         if (pmu->version > 1) {
> > -               pmu->global_ctrl_mask = ~((1ull << pmu->nr_arch_gp_counters)
> > - 1);
> > +               pmu->global_ctrl = (1ull << pmu->nr_arch_gp_counters) - 1;
> > +               pmu->global_ctrl_mask = ~pmu->global_ctrl;
> >                 pmu->global_status_mask = pmu->global_ctrl_mask;
> >         }
> > 
> > > > >        }
> > > > >          pmu->counter_bitmask[KVM_PMC_GP] = ((u64)1 << 48) - 1;
> > >