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

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Message-ID: <ZeYlEGORqeTPLK2_@google.com>
Date: Mon, 4 Mar 2024 11:46:24 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Sandipan Das <sandipan.das@....com>, Like Xu <like.xu.linux@...il.com>, pbonzini@...hat.com, 
	mizhang@...gle.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, 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.

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.

> 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;
> >