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

Commit Message

Mingwei Zhang Nov. 21, 2024, 6:53 p.m. UTC

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 <[e-mail]>
Co-developed-by: Jim Mattson <[e-mail]>
Signed-off-by: Jim Mattson <[e-mail]>
---
 arch/x86/kvm/svm/svm.c | 7 +++++++
 arch/x86/kvm/svm/svm.h | 2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

Patch

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;