[PATCH v6 44/44] KVM: VMX: Add mediated PMU support for CPUs without "save perf global ctrl"

Sean Christopherson
2025-12-06T00:18:55Z

Extend mediated PMU support for Intel CPUs without support for saving
PERF_GLOBAL_CONTROL into the guest VMCS field on VM-Exit, e.g. for Skylake
and its derivatives, as well as Icelake. While supporting CPUs without
VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL isn't completely trivial, it's not that
complex either. And not supporting such CPUs would mean not supporting 7+
years of Intel CPUs released in the past 10 years.

On VM-Exit, immediately propagate the saved PERF_GLOBAL_CTRL to the VMCS
as well as KVM's software cache so that KVM doesn't need to add full EXREG
tracking of PERF_GLOBAL_CTRL. In practice, the vast majority of VM-Exits
won't trigger software writes to guest PERF_GLOBAL_CTRL, so deferring the
VMWRITE to the next VM-Enter would only delay the inevitable without
batching/avoiding VMWRITEs.

Note! Take care to refresh VM_EXIT_MSR_STORE_COUNT on nested VM-Exit, as
it's unfortunately possible that KVM could recalculate MSR intercepts
while L2 is active, e.g. if userspace loads nested state and _then_ sets
PERF_CAPABILITIES. Eating the VMWRITE on every nested VM-Exit is
unfortunate, but that's a pre-existing problem and can/should be solved
separately, e.g. modifying the number of auto-load entries while L2 is
active is also uncommon on modern CPUs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/nested.c | 6 ++++-
arch/x86/kvm/vmx/pmu_intel.c | 7 -----
arch/x86/kvm/vmx/vmx.c | 52 ++++++++++++++++++++++++++++++++----
3 files changed, 52 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 614b789ecf16..1ee1edc8419d 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5142,7 +5142,11 @@ void __nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,

kvm_nested_vmexit_handle_ibrs(vcpu);

- /* Update any VMCS fields that might have changed while L2 ran */
+ /*
+ * Update any VMCS fields that might have changed while vmcs02 was the
+ * active VMCS. The tracking is per-vCPU, not per-VMCS.
+ */
+ vmcs_write32(VM_EXIT_MSR_STORE_COUNT, vmx->msr_autostore.nr);
vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 55249fa4db95..27eb76e6b6a0 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -777,13 +777,6 @@ static bool intel_pmu_is_mediated_pmu_supported(struct x86_pmu_capability *host_
if (WARN_ON_ONCE(!cpu_has_load_perf_global_ctrl()))
return false;

- /*
- * KVM doesn't yet support mediated PMU on CPUs without support for
- * saving PERF_GLOBAL_CTRL via a dedicated VMCS field.
- */
- if (!cpu_has_save_perf_global_ctrl())
- return false;
-
return true;
}

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6a17cb90eaf4..ba1262c3e3ff 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1204,6 +1204,17 @@ static bool update_transition_efer(struct vcpu_vmx *vmx)
return true;
}

+static void vmx_add_autostore_msr(struct vcpu_vmx *vmx, u32 msr)
+{
+ vmx_add_auto_msr(&vmx->msr_autostore, msr, 0, VM_EXIT_MSR_STORE_COUNT,
+ vmx->vcpu.kvm);
+}
+
+static void vmx_remove_autostore_msr(struct vcpu_vmx *vmx, u32 msr)
+{
+ vmx_remove_auto_msr(&vmx->msr_autostore, msr, VM_EXIT_MSR_STORE_COUNT);
+}
+
#ifdef CONFIG_X86_32
/*
* On 32-bit kernels, VM exits still load the FS and GS bases from the
@@ -4225,6 +4236,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)

static void vmx_recalc_pmu_msr_intercepts(struct kvm_vcpu *vcpu)
{
+ u64 vm_exit_controls_bits = VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL |
+ VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL;
bool has_mediated_pmu = kvm_vcpu_has_mediated_pmu(vcpu);
struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4234,12 +4247,19 @@ static void vmx_recalc_pmu_msr_intercepts(struct kvm_vcpu *vcpu)
if (!enable_mediated_pmu)
return;

+ if (!cpu_has_save_perf_global_ctrl()) {
+ vm_exit_controls_bits &= ~VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL;
+
+ if (has_mediated_pmu)
+ vmx_add_autostore_msr(vmx, MSR_CORE_PERF_GLOBAL_CTRL);
+ else
+ vmx_remove_autostore_msr(vmx, MSR_CORE_PERF_GLOBAL_CTRL);
+ }
+
vm_entry_controls_changebit(vmx, VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL,
has_mediated_pmu);

- vm_exit_controls_changebit(vmx, VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL |
- VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL,
- has_mediated_pmu);
+ vm_exit_controls_changebit(vmx, vm_exit_controls_bits, has_mediated_pmu);

for (i = 0; i < pmu->nr_arch_gp_counters; i++) {
vmx_set_intercept_for_msr(vcpu, MSR_IA32_PERFCTR0 + i,
@@ -7346,6 +7366,29 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
msrs[i].host);
}

+static void vmx_refresh_guest_perf_global_control(struct kvm_vcpu *vcpu)
+{
+ struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
+ struct vcpu_vmx *vmx = to_vmx(vcpu);
+
+ if (msr_write_intercepted(vmx, MSR_CORE_PERF_GLOBAL_CTRL))
+ return;
+
+ if (!cpu_has_save_perf_global_ctrl()) {
+ int slot = vmx_find_loadstore_msr_slot(&vmx->msr_autostore,
+ MSR_CORE_PERF_GLOBAL_CTRL);
+
+ if (WARN_ON_ONCE(slot < 0))
+ return;
+
+ pmu->global_ctrl = vmx->msr_autostore.val[slot].value;
+ vmcs_write64(GUEST_IA32_PERF_GLOBAL_CTRL, pmu->global_ctrl);
+ return;
+ }
+
+ pmu->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
+}
+
static void vmx_update_hv_timer(struct kvm_vcpu *vcpu, bool force_immediate_exit)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -7631,8 +7674,7 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)

vmx->loaded_vmcs->launched = 1;

- if (!msr_write_intercepted(vmx, MSR_CORE_PERF_GLOBAL_CTRL))
- vcpu_to_pmu(vcpu)->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
+ vmx_refresh_guest_perf_global_control(vcpu);

vmx_recover_nmi_blocking(vmx);
vmx_complete_interrupts(vmx);
--
2.52.0.223.gf5cc29aaa4-goog