KVM: x86: Remove unnecessary caching of KVM's PV CPUID base

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.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

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;
 
 	/*

arch/x86/kvm/cpuid.c

@@ -189,12 +189,7 @@ 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!
 	 */
@@ -252,28 +247,16 @@ 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;
 
@@ -504,7 +487,6 @@ 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