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

Mi, Dapeng
2025-12-09T06:31:28Z

On 12/8/2025 5:39 PM, Mi, Dapeng wrote:
> On 12/6/2025 8:17 AM, Sean Christopherson wrote:
>> 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);
> The change looks good to me, but I still have no bandwidth to test the code
> on Ice lake, I would go back after I finish the tests. Thanks.

Tested below test cases on Skylake and Sapphire Rapids servers for both
mediated vPMU and Perf based emulated vPMU, no issue is found. Thanks.

Kselftests: pmu_counters_test/pmu_event_filter_test/vmx_pmu_caps_test

KVM-unit-tests: pmu/pmu_lbr/pmu_pebs

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>

Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>

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