KVM: x86/pmu: Disable vPMU support on hybrid CPUs (host PMUs)

Disable KVM support for virtualizing PMUs on hosts with hybrid PMUs until
KVM gains a sane way to enumeration the hybrid vPMU to userspace and/or
gains a mechanism to let userspace opt-in to the dangers of exposing a
hybrid vPMU to KVM guests.  Virtualizing a hybrid PMU, or at least part of
a hybrid PMU, is possible, but it requires careful, deliberate
configuration from userspace.

E.g. to expose full functionality, vCPUs need to be pinned to pCPUs to
prevent migrating a vCPU between a big core and a little core, userspace
must enumerate a reasonable topology to the guest, and guest CPUID must be
curated per vCPU to enumerate accurate vPMU capabilities.

The last point is especially problematic, as KVM doesn't control which
pCPU it runs on when enumerating KVM's vPMU capabilities to userspace,
i.e. userspace can't rely on KVM_GET_SUPPORTED_CPUID in it's current form.

Alternatively, userspace could enable vPMU support by enumerating the
set of features that are common and coherent across all cores, e.g. by
filtering PMU events and restricting guest capabilities.  But again, that
requires userspace to take action far beyond reflecting KVM's supported
feature set into the guest.

For now, simply disable vPMU support on hybrid CPUs to avoid inducing
seemingly random #GPs in guests, and punt support for hybrid CPUs to a
future enabling effort.

Reported-by: Jianfeng Gao <[email protected]>
Cc: [email protected]
Cc: Andrew Cooper <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Andi Kleen <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: sean-jc

arch/x86/kvm/pmu.h

@@ -165,15 +165,27 @@ static inline void kvm_init_pmu_capability(void)
 {
 	bool is_intel = boot_cpu_data.x86_vendor == X86_VENDOR_INTEL;
 
-	perf_get_x86_pmu_capability(&kvm_pmu_cap);
-
-	 /*
-	  * For Intel, only support guest architectural pmu
-	  * on a host with architectural pmu.
-	  */
-	if ((is_intel && !kvm_pmu_cap.version) || !kvm_pmu_cap.num_counters_gp)
+	/*
+	 * Hybrid PMUs don't play nice with virtualization without careful
+	 * configuration by userspace, and KVM's APIs for reporting supported
+	 * vPMU features do not account for hybrid PMUs.  Disable vPMU support
+	 * for hybrid PMUs until KVM gains a way to let userspace opt-in.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_HYBRID_CPU))
 		enable_pmu = false;
 
+	if (enable_pmu) {
+		perf_get_x86_pmu_capability(&kvm_pmu_cap);
+
+		/*
+		 * For Intel, only support guest architectural pmu
+		 * on a host with architectural pmu.
+		 */
+		if ((is_intel && !kvm_pmu_cap.version) ||
+		    !kvm_pmu_cap.num_counters_gp)
+			enable_pmu = false;
+	}
+
 	if (!enable_pmu) {
 		memset(&kvm_pmu_cap, 0, sizeof(kvm_pmu_cap));
 		return;