[v2,29/49] KVM: x86: Remove unnecessary caching of KVM's PV CPUID base

Commit Message

Sean Christopherson May 17, 2024, 5:39 p.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, 7 insertions(+), 28 deletions(-)

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index aabf1648a56a..3003e99155e7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -858,7 +858,6 @@  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 93a7399dc0db..7290f91c422c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -269,28 +269,16 @@  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;
 
@@ -491,13 +479,6 @@  static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 	 * whether the supplied CPUID data is equal to what's already set.
 	 */
 	if (kvm_vcpu_has_run(vcpu)) {
-		/*
-		 * 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 KVM will reject the change.
-		 */
 		kvm_update_cpuid_runtime(vcpu);
 		kvm_apply_cpuid_pv_features_quirk(vcpu);
 
@@ -519,7 +500,6 @@  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