[PATCH V2 2/4] KVM: x86/pmu: Support Intel fixed counter 3 on mediated vPMU

Zide Chen
2026-04-23T17:55:00Z

From: Dapeng Mi <dapeng1.mi@linux.intel.com>

Starting with Ice Lake, Intel introduces fixed counter 3, which counts
TOPDOWN.SLOTS - the number of available slots for an unhalted logical
processor. It serves as the denominator for top-level metrics in the
Top-down Microarchitecture Analysis method.

Emulating this counter on legacy vPMU would require introducing a new
generic perf encoding for the Intel-specific TOPDOWN.SLOTS event in
order to call perf_get_hw_event_config(). This is undesirable as it
would pollute the generic perf event encoding.

Moreover, KVM does not intend to emulate IA32_PERF_METRICS in the
legacy vPMU model, and without IA32_PERF_METRICS, emulating this
counter has little practical value. Therefore, expose fixed counter
3 to guests only when mediated vPMU is enabled.

Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Co-developed-by: Zide Chen <zide.chen@intel.com>
Signed-off-by: Zide Chen <zide.chen@intel.com>
---
V2:
- Don't advertise fixed counter 3 to userspace if the host doesn't
support it.
---
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/kvm/cpuid.c | 9 +++++++--
arch/x86/kvm/pmu.c | 4 ++++
arch/x86/kvm/x86.c | 4 ++--
4 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c470e40a00aa..cb736a4c72ea 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -556,7 +556,7 @@ struct kvm_pmc {
#define KVM_MAX_NR_GP_COUNTERS KVM_MAX(KVM_MAX_NR_INTEL_GP_COUNTERS, \
KVM_MAX_NR_AMD_GP_COUNTERS)

-#define KVM_MAX_NR_INTEL_FIXED_COUNTERS 3
+#define KVM_MAX_NR_INTEL_FIXED_COUNTERS 4
#define KVM_MAX_NR_AMD_FIXED_COUNTERS 0
#define KVM_MAX_NR_FIXED_COUNTERS KVM_MAX(KVM_MAX_NR_INTEL_FIXED_COUNTERS, \
KVM_MAX_NR_AMD_FIXED_COUNTERS)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e69156b54cff..d87a26f740e5 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1505,7 +1505,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
break;
case 0xa: { /* Architectural Performance Monitoring */
union cpuid10_eax eax = { };
- union cpuid10_edx edx = { };
+ union cpuid10_edx edx = { }, host_edx;

if (!enable_pmu || !static_cpu_has(X86_FEATURE_ARCH_PERFMON)) {
entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
@@ -1516,9 +1516,14 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
eax.split.num_counters = kvm_pmu_cap.num_counters_gp;
eax.split.bit_width = kvm_pmu_cap.bit_width_gp;
eax.split.mask_length = kvm_pmu_cap.events_mask_len;
- edx.split.num_counters_fixed = kvm_pmu_cap.num_counters_fixed;
edx.split.bit_width_fixed = kvm_pmu_cap.bit_width_fixed;

+ /* Guest does not support non-contiguous fixed counters. */
+ host_edx = (union cpuid10_edx)entry->edx;
+ edx.split.num_counters_fixed =
+ min_t(int, kvm_pmu_cap.num_counters_fixed,
+ host_edx.split.num_counters_fixed);
+
if (kvm_pmu_cap.version)
edx.split.anythread_deprecated = 1;

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index e218352e3423..9ff4a6a9cd0b 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -148,12 +148,16 @@ void kvm_init_pmu_capability(struct kvm_pmu_ops *pmu_ops)
}

memcpy(&kvm_pmu_cap, &kvm_host_pmu, sizeof(kvm_host_pmu));
+
kvm_pmu_cap.version = min(kvm_pmu_cap.version, 2);
kvm_pmu_cap.num_counters_gp = min(kvm_pmu_cap.num_counters_gp,
pmu_ops->MAX_NR_GP_COUNTERS);
kvm_pmu_cap.num_counters_fixed = min(kvm_pmu_cap.num_counters_fixed,
KVM_MAX_NR_FIXED_COUNTERS);

+ if (!enable_mediated_pmu && kvm_pmu_cap.num_counters_fixed > 3)
+ kvm_pmu_cap.num_counters_fixed = 3;
+
kvm_pmu_eventsel.INSTRUCTIONS_RETIRED =
perf_get_hw_event_config(PERF_COUNT_HW_INSTRUCTIONS);
kvm_pmu_eventsel.BRANCH_INSTRUCTIONS_RETIRED =
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0a1b63c63d1a..604072d9354f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -360,7 +360,7 @@ static const u32 msrs_to_save_base[] = {

static const u32 msrs_to_save_pmu[] = {
MSR_ARCH_PERFMON_FIXED_CTR0, MSR_ARCH_PERFMON_FIXED_CTR1,
- MSR_ARCH_PERFMON_FIXED_CTR0 + 2,
+ MSR_ARCH_PERFMON_FIXED_CTR2, MSR_ARCH_PERFMON_FIXED_CTR3,
MSR_CORE_PERF_FIXED_CTR_CTRL, MSR_CORE_PERF_GLOBAL_STATUS,
MSR_CORE_PERF_GLOBAL_CTRL,
MSR_IA32_PEBS_ENABLE, MSR_IA32_DS_AREA, MSR_PEBS_DATA_CFG,
@@ -7756,7 +7756,7 @@ static void kvm_init_msr_lists(void)
{
unsigned i;

- BUILD_BUG_ON_MSG(KVM_MAX_NR_FIXED_COUNTERS != 3,
+ BUILD_BUG_ON_MSG(KVM_MAX_NR_FIXED_COUNTERS != 4,
"Please update the fixed PMCs in msrs_to_save_pmu[]");

num_msrs_to_save = 0;
--
2.54.0