KVM: x86: Explicitly do runtime CPUID updates "after" initial setup

Explicitly perform runtime CPUID adjustments as part of the "after set
CPUID" flow to guard against bugs where KVM consumes stale vCPU/CPUID
state during kvm_update_cpuid_runtime().  E.g. see commit 4736d85
("KVM: x86: Use actual kvm_cpuid.base for clearing KVM_FEATURE_PV_UNHALT").

Whacking each mole individually is not sustainable or robust, e.g. while
the aforemention commit fixed KVM's PV features, the same issue lurks for
Xen and Hyper-V features, Xen and Hyper-V simply don't have any runtime
features (though spoiler alert, neither should KVM).

Updating runtime features in the "full" path will also simplify adding a
snapshot of the guest's capabilities, i.e. of caching the intersection of
guest CPUID and kvm_cpu_caps (modulo a few edge cases).

Link: https://lore.kernel.org/r/20241128013424.4096668-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/cpuid.c

@@ -178,13 +178,27 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
 	return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);
 }
 
+static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
+				       int nent);
+
 /* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
 static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 				 int nent)
 {
 	struct kvm_cpuid_entry2 *orig;
 	int i;
 
+	/*
+	 * 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.
+	 */
+	__kvm_update_cpuid_runtime(vcpu, e2, nent);
+
 	if (nent != vcpu->arch.cpuid_nent)
 		return -EINVAL;
 
@@ -369,6 +383,8 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	bitmap_zero(vcpu->arch.governed_features.enabled,
 		    KVM_MAX_NR_GOVERNED_FEATURES);
 
+	kvm_update_cpuid_runtime(vcpu);
+
 	/*
 	 * If TDP is enabled, let the guest use GBPAGES if they're supported in
 	 * hardware.  The hardware page walker doesn't let KVM disable GBPAGES,
@@ -450,8 +466,6 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 {
 	int r;
 
-	__kvm_update_cpuid_runtime(vcpu, e2, nent);
-
 	/*
 	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
 	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't