KVM: x86/pmu: Disallow "fast" RDPMC for architectural Intel PMUs

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.

Opportunistically WARN if KVM ever actually tries to complete RDPMC for a
non-architectural PMU, and drop the non-existent "support" for fast RDPMC,
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")
Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 18 insertions, 4 deletions

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 03bd188b5754..5a5dfae6055c 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -27,7 +27,6 @@
  * "fast" reads, whereas the "type" is an explicit value.
  */
 #define INTEL_RDPMC_FIXED	INTEL_PMC_FIXED_RDPMC_BASE
-#define INTEL_RDPMC_FAST	BIT(31)
 
 #define MSR_PMC_FULL_WIDTH_BIT      (MSR_IA32_PMC0 - MSR_IA32_PERFCTR0)
 
@@ -72,10 +71,25 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
 	struct kvm_pmc *counters;
 	unsigned int num_counters;
 
-	if (idx & INTEL_RDPMC_FAST)
-		*mask &= GENMASK_ULL(31, 0);
+	/*
+	 * The encoding of ECX for RDPMC is different for architectural versus
+	 * non-architecturals PMUs (PMUs with version '0').  For architectural
+	 * PMUs, bits 31:16 specify the PMC type and bits 15:0 specify the PMC
+	 * index.  For non-architectural PMUs, bit 31 is a "fast" flag, and
+	 * bits 30:0 specify the PMC index.
+	 *
+	 * Yell and reject attempts to read PMCs for a non-architectural PMU,
+	 * as KVM doesn't support such PMUs.
+	 */
+	if (WARN_ON_ONCE(!pmu->version))
+		return NULL;
 
-	idx &= ~(INTEL_RDPMC_FIXED | 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.
+	 */
+	idx &= ~INTEL_RDPMC_FIXED;
 	if (fixed) {
 		counters = pmu->fixed_counters;
 		num_counters = pmu->nr_arch_fixed_counters;