On Wed, Nov 20, 2024, Dapeng Mi wrote:
>
> On 11/19/2024 10:54 PM, Sean Christopherson wrote:
> > On Thu, Aug 01, 2024, Mingwei Zhang wrote:
> >> Plumb through pass-through PMU setting from kvm->arch into kvm_pmu on each
> >> vcpu created. Note that enabling PMU is decided by VMM when it sets the
> >> CPUID bits exposed to guest VM. So plumb through the enabling for each pmu
> >> in intel_pmu_refresh().
> > Why? As with the per-VM snapshot, I see zero reason for this to exist, it's
> > simply:
> >
> > kvm->arch.enable_pmu && enable_mediated_pmu && pmu->version;
> >
> > And in literally every correct usage of pmu->passthrough, kvm->arch.enable_pmu
> > and pmu->version have been checked (though implicitly), i.e. KVM can check
> > enable_mediated_pmu and nothing else.
>
> Ok, too many passthrough_pmu flags indeed confuse readers. Besides these
> dependencies, mediated vPMU also depends on lapic_in_kernel(). We need to
> set enable_mediated_pmu to false as well if lapic_in_kernel() returns false.
No, just kill the entire vPMU.
Also, the need for an in-kernel APIC isn't unique to the mediated PMU. KVM simply
drops PMIs if there's no APIC.
If we're feeling lucky, we could try a breaking change like so:
diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index fcd188cc389a..bb08155f6198 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -817,7 +817,7 @@ void kvm_pmu_refresh(struct kvm_vcpu *vcpu)
pmu->pebs_data_cfg_mask = ~0ull;
bitmap_zero(pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX);
- if (!vcpu->kvm->arch.enable_pmu)
+ if (!vcpu->kvm->arch.enable_pmu || !lapic_in_kernel(vcpu))
return;
static_call(kvm_x86_pmu_refresh)(vcpu);
If we don't want to risk breaking weird setups, we could restrict the behavior
to the mediated PMU being enabled:
diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index fcd188cc389a..bc9673190574 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -817,7 +817,8 @@ void kvm_pmu_refresh(struct kvm_vcpu *vcpu)
pmu->pebs_data_cfg_mask = ~0ull;
bitmap_zero(pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX);
- if (!vcpu->kvm->arch.enable_pmu)
+ if (!vcpu->kvm->arch.enable_pmu ||
+ (!lapic_in_kernel(vcpu) && enable_mediated_pmu))
return;
static_call(kvm_x86_pmu_refresh)(vcpu);
