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Message-ID: <CALMp9eRHrEZX4-JWyGXNRvafU2dNbfa6ZjT5HhrDBYujGYEvaw@mail.gmail.com>
Date: Thu, 13 Mar 2025 13:40:54 -0700
From: Jim Mattson <jmattson@...gle.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86: Provide a capability to disable APERF/MPERF
read intercepts
On Mon, Feb 24, 2025 at 4:47 PM Jim Mattson <jmattson@...gle.com> wrote:
>
> Allow a guest to read the physical IA32_APERF and IA32_MPERF MSRs
> without interception.
>
> The IA32_APERF and IA32_MPERF MSRs are not virtualized. Writes are not
> handled at all. The MSR values are not zeroed on vCPU creation, saved
> on suspend, or restored on resume. No accommodation is made for
> processor migration or for sharing a logical processor with other
> tasks. No adjustments are made for non-unit TSC multipliers. The MSRs
> do not account for time the same way as the comparable PMU events,
> whether the PMU is virtualized by the traditional emulation method or
> the new mediated pass-through approach.
>
> Nonetheless, in a properly constrained environment, this capability
> can be combined with a guest CPUID table that advertises support for
> CPUID.6:ECX.APERFMPERF[bit 0] to induce a Linux guest to report the
> effective physical CPU frequency in /proc/cpuinfo. Moreover, there is
> no performance cost for this capability.
>
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
> Documentation/virt/kvm/api.rst | 1 +
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/svm/svm.c | 7 +++++++
> arch/x86/kvm/svm/svm.h | 2 +-
> arch/x86/kvm/vmx/vmx.c | 4 ++++
> arch/x86/kvm/x86.c | 7 +++++--
> arch/x86/kvm/x86.h | 5 +++++
> include/uapi/linux/kvm.h | 1 +
> tools/include/uapi/linux/kvm.h | 4 +++-
> 9 files changed, 28 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 2b52eb77e29c..6431cd33f06a 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7684,6 +7684,7 @@ Valid bits in args[0] are::
> #define KVM_X86_DISABLE_EXITS_HLT (1 << 1)
> #define KVM_X86_DISABLE_EXITS_PAUSE (1 << 2)
> #define KVM_X86_DISABLE_EXITS_CSTATE (1 << 3)
> + #define KVM_X86_DISABLE_EXITS_APERFMPERF (1 << 4)
>
> Enabling this capability on a VM provides userspace with a way to no
> longer intercept some instructions for improved latency in some
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0b7af5902ff7..53de91fccc20 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1380,6 +1380,7 @@ struct kvm_arch {
> bool hlt_in_guest;
> bool pause_in_guest;
> bool cstate_in_guest;
> + bool aperfmperf_in_guest;
>
> unsigned long irq_sources_bitmap;
> s64 kvmclock_offset;
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index a713c803a3a3..5ebcbff341bc 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -111,6 +111,8 @@ static const struct svm_direct_access_msrs {
> { .index = MSR_IA32_CR_PAT, .always = false },
> { .index = MSR_AMD64_SEV_ES_GHCB, .always = true },
> { .index = MSR_TSC_AUX, .always = false },
> + { .index = MSR_IA32_APERF, .always = false },
> + { .index = MSR_IA32_MPERF, .always = false },
> { .index = X2APIC_MSR(APIC_ID), .always = false },
> { .index = X2APIC_MSR(APIC_LVR), .always = false },
> { .index = X2APIC_MSR(APIC_TASKPRI), .always = false },
> @@ -1359,6 +1361,11 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
> if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SPEC_CTRL, 1, 1);
>
> + if (kvm_aperfmperf_in_guest(vcpu->kvm)) {
> + set_msr_interception(vcpu, svm->msrpm, MSR_IA32_APERF, 1, 0);
> + set_msr_interception(vcpu, svm->msrpm, MSR_IA32_MPERF, 1, 0);
> + }
> +
> if (kvm_vcpu_apicv_active(vcpu))
> avic_init_vmcb(svm, vmcb);
>
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 9d7cdb8fbf87..3ee2b7e07395 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -44,7 +44,7 @@ static inline struct page *__sme_pa_to_page(unsigned long pa)
> #define IOPM_SIZE PAGE_SIZE * 3
> #define MSRPM_SIZE PAGE_SIZE * 2
>
> -#define MAX_DIRECT_ACCESS_MSRS 48
> +#define MAX_DIRECT_ACCESS_MSRS 50
> #define MSRPM_OFFSETS 32
> extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
> extern bool npt_enabled;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6c56d5235f0f..88a555328932 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7597,6 +7597,10 @@ int vmx_vcpu_create(struct kvm_vcpu *vcpu)
> vmx_disable_intercept_for_msr(vcpu, MSR_CORE_C6_RESIDENCY, MSR_TYPE_R);
> vmx_disable_intercept_for_msr(vcpu, MSR_CORE_C7_RESIDENCY, MSR_TYPE_R);
> }
> + if (kvm_aperfmperf_in_guest(vcpu->kvm)) {
> + vmx_disable_intercept_for_msr(vcpu, MSR_IA32_APERF, MSR_TYPE_R);
> + vmx_disable_intercept_for_msr(vcpu, MSR_IA32_MPERF, MSR_TYPE_R);
> + }
>
> vmx->loaded_vmcs = &vmx->vmcs01;
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 02159c967d29..98f3df24ac9a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4533,7 +4533,7 @@ static inline bool kvm_can_mwait_in_guest(void)
>
> static u64 kvm_get_allowed_disable_exits(void)
> {
> - u64 r = KVM_X86_DISABLE_EXITS_PAUSE;
> + u64 r = KVM_X86_DISABLE_EXITS_PAUSE | KVM_X86_DISABLE_EXITS_APERFMPERF;
>
> if (!mitigate_smt_rsb) {
> r |= KVM_X86_DISABLE_EXITS_HLT |
> @@ -6543,7 +6543,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>
> if (!mitigate_smt_rsb && boot_cpu_has_bug(X86_BUG_SMT_RSB) &&
> cpu_smt_possible() &&
> - (cap->args[0] & ~KVM_X86_DISABLE_EXITS_PAUSE))
> + (cap->args[0] & ~(KVM_X86_DISABLE_EXITS_PAUSE |
> + KVM_X86_DISABLE_EXITS_APERFMPERF)))
> pr_warn_once(SMT_RSB_MSG);
>
> if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
> @@ -6554,6 +6555,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> kvm->arch.hlt_in_guest = true;
> if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
> kvm->arch.cstate_in_guest = true;
> + if (cap->args[0] & KVM_X86_DISABLE_EXITS_APERFMPERF)
> + kvm->arch.aperfmperf_in_guest = true;
> r = 0;
> disable_exits_unlock:
> mutex_unlock(&kvm->lock);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 91e50a513100..0c3ac99454e5 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -488,6 +488,11 @@ static inline bool kvm_cstate_in_guest(struct kvm *kvm)
> return kvm->arch.cstate_in_guest;
> }
>
> +static inline bool kvm_aperfmperf_in_guest(struct kvm *kvm)
> +{
> + return kvm->arch.aperfmperf_in_guest;
> +}
> +
> static inline bool kvm_notify_vmexit_enabled(struct kvm *kvm)
> {
> return kvm->arch.notify_vmexit_flags & KVM_X86_NOTIFY_VMEXIT_ENABLED;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 45e6d8fca9b9..b4a4eb52f6df 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -617,6 +617,7 @@ struct kvm_ioeventfd {
> #define KVM_X86_DISABLE_EXITS_HLT (1 << 1)
> #define KVM_X86_DISABLE_EXITS_PAUSE (1 << 2)
> #define KVM_X86_DISABLE_EXITS_CSTATE (1 << 3)
> +#define KVM_X86_DISABLE_EXITS_APERFMPERF (1 << 4)
>
> /* for KVM_ENABLE_CAP */
> struct kvm_enable_cap {
> diff --git a/tools/include/uapi/linux/kvm.h b/tools/include/uapi/linux/kvm.h
> index 502ea63b5d2e..9b60f0509cdc 100644
> --- a/tools/include/uapi/linux/kvm.h
> +++ b/tools/include/uapi/linux/kvm.h
> @@ -617,10 +617,12 @@ struct kvm_ioeventfd {
> #define KVM_X86_DISABLE_EXITS_HLT (1 << 1)
> #define KVM_X86_DISABLE_EXITS_PAUSE (1 << 2)
> #define KVM_X86_DISABLE_EXITS_CSTATE (1 << 3)
> +#define KVM_X86_DISABLE_EXITS_APERFMPERF (1 << 4)
> #define KVM_X86_DISABLE_VALID_EXITS (KVM_X86_DISABLE_EXITS_MWAIT | \
> KVM_X86_DISABLE_EXITS_HLT | \
> KVM_X86_DISABLE_EXITS_PAUSE | \
> - KVM_X86_DISABLE_EXITS_CSTATE)
> + KVM_X86_DISABLE_EXITS_CSTATE | \
> + KVM_X86_DISABLE_EXITS_APERFMPERF)
>
> /* for KVM_ENABLE_CAP */
> struct kvm_enable_cap {
> --
> 2.48.1.658.g4767266eb4-goog
>
Ping.
Any thoughts?
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