[RFC PATCH 26/41] KVM: x86/pmu: Add host_perf_cap field in kvm_caps to record host PMU capability

[Xiong Zhang]

From: Mingwei Zhang <mizhang@xxxxxxxxxx>

Add host_perf_cap field in kvm_caps to record host PMU capability. This
helps KVM recognize the PMU capability difference between host and guest.
This awareness improves performance in PMU context switch. In particular,
KVM will need to zero out all MSRs that guest PMU does not use but host PMU
does use. Having the host PMU feature set cached in host_perf_cap in
kvm_caps structure saves a rdmsrl() to IA32_PERF_CAPABILITY MSR on each PMU
context switch. In addition, this is more convenient approach than open
another API on the host perf subsystem side.

Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/vmx.c | 17 +++++++++--------
 arch/x86/kvm/x86.h     |  1 +
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 349954f90fe9..50100954cd92 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7896,32 +7896,33 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	vmx_update_exception_bitmap(vcpu);
 }
 
-static u64 vmx_get_perf_capabilities(void)
+static void vmx_get_perf_capabilities(void)
 {
 	u64 perf_cap = PMU_CAP_FW_WRITES;
 	struct x86_pmu_lbr lbr;
-	u64 host_perf_cap = 0;
+
+	kvm_caps.host_perf_cap = 0;
 
 	if (!enable_pmu)
-		return 0;
+		return;
 
 	if (boot_cpu_has(X86_FEATURE_PDCM))
-		rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
+		rdmsrl(MSR_IA32_PERF_CAPABILITIES, kvm_caps.host_perf_cap);
 
 	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR) &&
 	    !enable_passthrough_pmu) {
 		x86_perf_get_lbr(&lbr);
 		if (lbr.nr)
-			perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
+			perf_cap |= kvm_caps.host_perf_cap & PMU_CAP_LBR_FMT;
 	}
 
 	if (vmx_pebs_supported() && !enable_passthrough_pmu) {
-		perf_cap |= host_perf_cap & PERF_CAP_PEBS_MASK;
+		perf_cap |= kvm_caps.host_perf_cap & PERF_CAP_PEBS_MASK;
 		if ((perf_cap & PERF_CAP_PEBS_FORMAT) < 4)
 			perf_cap &= ~PERF_CAP_PEBS_BASELINE;
 	}
 
-	return perf_cap;
+	kvm_caps.supported_perf_cap = perf_cap;
 }
 
 static __init void vmx_set_cpu_caps(void)
@@ -7946,7 +7947,7 @@ static __init void vmx_set_cpu_caps(void)
 
 	if (!enable_pmu)
 		kvm_cpu_cap_clear(X86_FEATURE_PDCM);
-	kvm_caps.supported_perf_cap = vmx_get_perf_capabilities();
+	vmx_get_perf_capabilities();
 
 	if (!enable_sgx) {
 		kvm_cpu_cap_clear(X86_FEATURE_SGX);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 38b73e98eae9..a29eb0469d7e 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -28,6 +28,7 @@ struct kvm_caps {
 	u64 supported_mce_cap;
 	u64 supported_xcr0;
 	u64 supported_xss;
+	u64 host_perf_cap;
 	u64 supported_perf_cap;
 };
 
-- 
2.34.1