[PATCH v8 08/26] KVM: x86/pmu: Disallow "fast" RDPMC for architectural Intel PMUs

[Sean Christopherson]

Inject #GP on RDPMC if the "fast" flag is set for architectural Intel
PMUs, i.e. if the PMU version is non-zero.  Per Intel's SDM, and confirmed
on bare metal, the "fast" flag is supported only for non-architectural
PMUs, and is reserved for architectural PMUs.

  If the processor does not support architectural performance monitoring
  (CPUID.0AH:EAX[7:0]=0), ECX[30:0] specifies the index of the PMC to be
  read. Setting ECX[31] selects “fast” read mode if supported. In this mode,
  RDPMC returns bits 31:0 of the PMC in EAX while clearing EDX to zero.

  If the processor does support architectural performance monitoring
  (CPUID.0AH:EAX[7:0] ≠ 0), ECX[31:16] specifies type of PMC while ECX[15:0]
  specifies the index of the PMC to be read within that type. The following
  PMC types are currently defined:
  — General-purpose counters use type 0. The index x (to read IA32_PMCx)
    must be less than the value enumerated by CPUID.0AH.EAX[15:8] (thus
    ECX[15:8] must be zero).
  — Fixed-function counters use type 4000H. The index x (to read
    IA32_FIXED_CTRx) can be used if either CPUID.0AH.EDX[4:0] > x or
    CPUID.0AH.ECX[x] = 1 (thus ECX[15:5] must be 0).
  — Performance metrics use type 2000H. This type can be used only if
    IA32_PERF_CAPABILITIES.PERF_METRICS_AVAILABLE[bit 15]=1. For this type,
    the index in ECX[15:0] is implementation specific.

WARN if KVM ever actually tries to complete RDPMC for a non-architectural
PMU as KVM doesn't support such PMUs, i.e. kvm_pmu_rdpmc() should reject
the RDPMC before getting to the Intel code.

Fixes: f5132b01386b ("KVM: Expose a version 2 architectural PMU to a guests")
Fixes: 67f4d4288c35 ("KVM: x86: rdpmc emulation checks the counter incorrectly")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/pmu_intel.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index c6ea128ea7c8..80255f86072e 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -61,7 +61,19 @@ static struct kvm_pmc *intel_pmc_idx_to_pmc(struct kvm_pmu *pmu, int pmc_idx)
 
 static u32 intel_rdpmc_get_masked_idx(struct kvm_pmu *pmu, u32 idx)
 {
-	return idx & ~(INTEL_RDPMC_FIXED | INTEL_RDPMC_FAST);
+	/*
+	 * Fast RDPMC is only supported on non-architectural PMUs, which KVM
+	 * doesn't support.
+	 */
+	if (WARN_ON_ONCE(!pmu->version))
+		return idx & ~INTEL_RDPMC_FAST;
+
+	/*
+	 * Fixed PMCs are supported on all architectural PMUs.  Note, KVM only
+	 * emulates fixed PMCs for PMU v2+, but the flag itself is still valid,
+	 * i.e. let RDPMC fail due to accessing a non-existent counter.
+	 */
+	return idx & ~INTEL_RDPMC_FIXED;
 }
 
 static bool intel_is_valid_rdpmc_ecx(struct kvm_vcpu *vcpu, unsigned int idx)
-- 
2.42.0.869.gea05f2083d-goog