[v3,33/57] KVM: x86: Remove unnecessary caching of KVM's PV CPUID base

Commit Message

Sean Christopherson Nov. 28, 2024, 1:34 a.m. UTC

Now that KVM only searches for KVM's PV CPUID base when userspace sets
guest CPUID, drop the cache and simply do the search every time.

Practically speaking, this is a nop except for situations where userspace
sets CPUID _after_ running the vCPU, which is anything but a hot path,
e.g. QEMU does so only when hotplugging a vCPU.  And on the flip side,
caching guest CPUID information, especially information that is used to
query/modify _other_ CPUID state, is inherently dangerous as it's all too
easy to use stale information, i.e. KVM should only cache CPUID state when
the performance and/or programming benefits justify it.

Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/include/asm/kvm_host.h |  1 -
 arch/x86/kvm/cpuid.c            | 34 ++++++++-------------------------
 2 files changed, 8 insertions(+), 27 deletions(-)

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e159e44a6a1b..f076df9f18be 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -854,7 +854,6 @@  struct kvm_vcpu_arch {
 
 	int cpuid_nent;
 	struct kvm_cpuid_entry2 *cpuid_entries;
-	struct kvm_hypervisor_cpuid kvm_cpuid;
 	bool is_amd_compatible;
 
 	/*
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 3ba0e6a67823..b402b9f59cbb 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -168,12 +168,7 @@  static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2
 
 	/*
 	 * Apply runtime CPUID updates to the incoming CPUID entries to avoid
-	 * false positives due mismatches on KVM-owned feature flags.  Note,
-	 * runtime CPUID updates may consume other CPUID-driven vCPU state,
-	 * e.g. KVM or Xen CPUID bases.  Updating runtime state before full
-	 * CPUID processing is functionally correct only because any change in
-	 * CPUID is disallowed, i.e. using stale data is ok because the below
-	 * checks will reject the change.
+	 * false positives due mismatches on KVM-owned feature flags.
 	 *
 	 * Note!  @e2 and @nent track the _old_ CPUID entries!
 	 */
@@ -231,28 +226,16 @@  static struct kvm_hypervisor_cpuid kvm_get_hypervisor_cpuid(struct kvm_vcpu *vcp
 					  vcpu->arch.cpuid_nent, sig);
 }
 
-static struct kvm_cpuid_entry2 *__kvm_find_kvm_cpuid_features(struct kvm_cpuid_entry2 *entries,
-							      int nent, u32 kvm_cpuid_base)
-{
-	return cpuid_entry2_find(entries, nent, kvm_cpuid_base | KVM_CPUID_FEATURES,
-				 KVM_CPUID_INDEX_NOT_SIGNIFICANT);
-}
-
-static struct kvm_cpuid_entry2 *kvm_find_kvm_cpuid_features(struct kvm_vcpu *vcpu)
-{
-	u32 base = vcpu->arch.kvm_cpuid.base;
-
-	if (!base)
-		return NULL;
-
-	return __kvm_find_kvm_cpuid_features(vcpu->arch.cpuid_entries,
-					     vcpu->arch.cpuid_nent, base);
-}
-
 static u32 kvm_apply_cpuid_pv_features_quirk(struct kvm_vcpu *vcpu)
 {
-	struct kvm_cpuid_entry2 *best = kvm_find_kvm_cpuid_features(vcpu);
+	struct kvm_hypervisor_cpuid kvm_cpuid;
+	struct kvm_cpuid_entry2 *best;
 
+	kvm_cpuid = kvm_get_hypervisor_cpuid(vcpu, KVM_SIGNATURE);
+	if (!kvm_cpuid.base)
+		return 0;
+
+	best = kvm_find_cpuid_entry(vcpu, kvm_cpuid.base | KVM_CPUID_FEATURES);
 	if (!best)
 		return 0;
 
@@ -483,7 +466,6 @@  static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	if (r)
 		goto err;
 
-	vcpu->arch.kvm_cpuid = kvm_get_hypervisor_cpuid(vcpu, KVM_SIGNATURE);
 #ifdef CONFIG_KVM_XEN
 	vcpu->arch.xen.cpuid = kvm_get_hypervisor_cpuid(vcpu, XEN_SIGNATURE);
 #endif