KVM: x86: Clear PV_UNHALT for !HLT-exiting only when userspace sets CPUID

Now that KVM disallows disabling HLT-exiting after vCPUs have been created,
i.e. now that it's impossible for kvm_hlt_in_guest() to change while vCPUs
are running, apply KVM's PV_UNHALT quirk only when userspace is setting
guest CPUID.

Opportunistically rename the helper to make it clear that KVM's behavior
is a quirk that should never have been added.  KVM's documentation
explicitly states that userspace should not advertise PV_UNHALT if
HLT-exiting is disabled, but for unknown reasons, commit caa057a2cad6
("KVM: X86: Provide a capability to disable HLT intercepts") didn't stop
at documenting the requirement and also massaged the incoming guest CPUID.

Unfortunately, it's quite likely that userspace has come to rely on KVM's
behavior, i.e. the code can't simply be deleted.  The only reason KVM
doesn't have an "official" quirk is that there is no known use case where
disabling the quirk would make sense, i.e. letting userspace disable the
quirk would further increase KVM's burden without any benefit.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-33-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 11 insertions, 17 deletions

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 6b56ed325ec4..ebc7cf0ac0bb 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -178,6 +178,8 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
 	return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);
 }
 
+static u32 kvm_apply_cpuid_pv_features_quirk(struct kvm_vcpu *vcpu);
+
 /* 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)
@@ -197,6 +199,7 @@ static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2
 	 * Note!  @e2 and @nent track the _old_ CPUID entries!
 	 */
 	kvm_update_cpuid_runtime(vcpu);
+	kvm_apply_cpuid_pv_features_quirk(vcpu);
 
 	if (nent != vcpu->arch.cpuid_nent)
 		return -EINVAL;
@@ -267,18 +270,17 @@ static struct kvm_cpuid_entry2 *kvm_find_kvm_cpuid_features(struct kvm_vcpu *vcp
 					     vcpu->arch.cpuid_nent, base);
 }
 
-static void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
+static u32 kvm_apply_cpuid_pv_features_quirk(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best = kvm_find_kvm_cpuid_features(vcpu);
 
-	vcpu->arch.pv_cpuid.features = 0;
+	if (!best)
+		return 0;
 
-	/*
-	 * save the feature bitmap to avoid cpuid lookup for every PV
-	 * operation
-	 */
-	if (best)
-		vcpu->arch.pv_cpuid.features = best->eax;
+	if (kvm_hlt_in_guest(vcpu->kvm))
+		best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
+
+	return best->eax;
 }
 
 /*
@@ -300,7 +302,6 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
 				       int nent)
 {
 	struct kvm_cpuid_entry2 *best;
-	struct kvm_hypervisor_cpuid kvm_cpuid;
 
 	best = cpuid_entry2_find(entries, nent, 1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
 	if (best) {
@@ -327,13 +328,6 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
 		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
 		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
 
-	kvm_cpuid = __kvm_get_hypervisor_cpuid(entries, nent, KVM_SIGNATURE);
-	if (kvm_cpuid.base) {
-		best = __kvm_find_kvm_cpuid_features(entries, nent, kvm_cpuid.base);
-		if (kvm_hlt_in_guest(vcpu->kvm) && best)
-			best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
-	}
-
 	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)) {
 		best = cpuid_entry2_find(entries, nent, 0x1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
 		if (best)
@@ -417,7 +411,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	vcpu->arch.guest_supported_xcr0 =
 		cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
 
-	kvm_update_pv_runtime(vcpu);
+	vcpu->arch.pv_cpuid.features = kvm_apply_cpuid_pv_features_quirk(vcpu);
 
 	vcpu->arch.is_amd_compatible = guest_cpuid_is_amd_or_hygon(vcpu);
 	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);