[RFC PATCH 21/22] KVM: SVM: Pass through guest reads of IA32_[AM]PERF

Mingwei Zhang
2024-11-21T18:53:54Z

Allow direct guest rdmsr access to IA32_APERF and IA32_MPERF while
continuing to intercept guest wrmsr. Direct read access reduces overhead
for guests that poll these MSRs frequently (e.g. every scheduler tick),
while write interception remains necessary to maintain proper host
offset tracking.

Signed-off-by: Mingwei Zhang <mizhang@google.com>
Co-developed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
---
arch/x86/kvm/svm/svm.c | 7 +++++++
arch/x86/kvm/svm/svm.h | 2 +-
2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 9df3e1e5ae81a..332947e0e9670 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -142,6 +142,8 @@ static const struct svm_direct_access_msrs {
{ .index = X2APIC_MSR(APIC_TMICT), .always = false },
{ .index = X2APIC_MSR(APIC_TMCCT), .always = false },
{ .index = X2APIC_MSR(APIC_TDCR), .always = false },
+ { .index = MSR_IA32_APERF, .always = false },
+ { .index = MSR_IA32_MPERF, .always = false },
{ .index = MSR_INVALID, .always = false },
};

@@ -1231,6 +1233,11 @@ static inline void init_vmcb_after_set_cpuid(struct kvm_vcpu *vcpu)
set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_EIP, 1, 1);
set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_ESP, 1, 1);
}
+
+ set_msr_interception(vcpu, svm->msrpm, MSR_IA32_APERF,
+ guest_can_use(vcpu, X86_FEATURE_APERFMPERF), 0);
+ set_msr_interception(vcpu, svm->msrpm, MSR_IA32_MPERF,
+ guest_can_use(vcpu, X86_FEATURE_APERFMPERF), 0);
}

static void init_vmcb(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 43fa6a16eb191..5ae5a13b9771a 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;
--
2.47.0.371.ga323438b13-goog