[PATCH 1/2] KVM: x86: Add CPUID cache for frequent X86_FEATURE_* guest lookups

[Sean Christopherson]

Implement a small "cache" for expediting guest_cpuid_has() lookups of
frequently used features.  Guest CPUID lookups are slow, especially if
the associated leaf has no entry, as KVM uses a linear walk of all CPUID
entries to find the associated leaf, e.g. adding a guest_cpuid_has()
lookup in the VM-Enter path is slow enough that it shows up on perf
traces.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/cpuid.c            |  9 +++++++++
 arch/x86/kvm/cpuid.h            | 28 ++++++++++++++++++++++++++--
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e8281d64a431..8cdb5c46815d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -759,6 +759,7 @@ struct kvm_vcpu_arch {
 	int cpuid_nent;
 	struct kvm_cpuid_entry2 *cpuid_entries;
 	u32 kvm_cpuid_base;
+	u32 kvm_cpuid_x86_feature_cache;
 
 	u64 reserved_gpa_bits;
 	int maxphyaddr;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 75dcf7a72605..27b25fdb4335 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -377,6 +377,12 @@ u64 kvm_vcpu_reserved_gpa_bits_raw(struct kvm_vcpu *vcpu)
 	return rsvd_bits(cpuid_maxphyaddr(vcpu), 63);
 }
 
+#define kvm_cpuid_cache_update(__vcpu, x86_feature)						\
+do {												\
+	if (__guest_cpuid_has(__vcpu, x86_feature))						\
+		(__vcpu)->arch.kvm_cpuid_x86_feature_cache |= BIT(KVM_CACHED_ ## x86_feature);	\
+} while (0)
+
 static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
                         int nent)
 {
@@ -412,6 +418,9 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	vcpu->arch.cpuid_entries = e2;
 	vcpu->arch.cpuid_nent = nent;
 
+	/* Update the cache before doing anything else. */
+	vcpu->arch.kvm_cpuid_x86_feature_cache = 0;
+
 	kvm_update_kvm_cpuid_base(vcpu);
 	kvm_vcpu_after_set_cpuid(vcpu);
 
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index b1658c0de847..49009d16022a 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -85,8 +85,21 @@ static __always_inline u32 *guest_cpuid_get_register(struct kvm_vcpu *vcpu,
 	return __cpuid_entry_get_reg(entry, cpuid.reg);
 }
 
-static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu,
-					    unsigned int x86_feature)
+enum kvm_cpuid_cached_feature {
+	NR_KVM_CACHED_X86_FEATURES,
+};
+
+static __always_inline int guest_cpuid_get_cache_bit(unsigned int x86_feature)
+{
+	int cache_bytes = sizeof((struct kvm_vcpu_arch *)0)->kvm_cpuid_x86_feature_cache;
+
+	BUILD_BUG_ON(NR_KVM_CACHED_X86_FEATURES > cache_bytes * BITS_PER_BYTE);
+
+	return NR_KVM_CACHED_X86_FEATURES;
+}
+
+static __always_inline bool __guest_cpuid_has(struct kvm_vcpu *vcpu,
+					      unsigned int x86_feature)
 {
 	u32 *reg;
 
@@ -97,6 +110,17 @@ static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu,
 	return *reg & __feature_bit(x86_feature);
 }
 
+static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu,
+					    unsigned int x86_feature)
+{
+	int cache_bit = guest_cpuid_get_cache_bit(x86_feature);
+
+	if (cache_bit != NR_KVM_CACHED_X86_FEATURES)
+		return vcpu->arch.kvm_cpuid_x86_feature_cache & BIT(cache_bit);
+
+	return __guest_cpuid_has(vcpu, x86_feature);
+}
+
 static __always_inline void guest_cpuid_clear(struct kvm_vcpu *vcpu,
 					      unsigned int x86_feature)
 {

base-commit: 1a4d88a361af4f2e91861d632c6a1fe87a9665c2
-- 
2.37.1.455.g008518b4e5-goog


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	filename="0002-KVM-x86-Cache-guest-CPUID-s-PDMC-for-fast-lookup.patch"