KVM: x86: Use actual kvm_cpuid.base for clearing KVM_FEATURE_PV_UNHALT

Commit ee3a5f9 ("KVM: x86: Do runtime CPUID update before updating
vcpu->arch.cpuid_entries") moved tweaking of the supplied CPUID
data earlier in kvm_set_cpuid() but __kvm_update_cpuid_runtime() actually
uses 'vcpu->arch.kvm_cpuid' (though __kvm_find_kvm_cpuid_features()) which
gets set later in kvm_set_cpuid(). In some cases, e.g. when kvm_set_cpuid()
is called for the first time and 'vcpu->arch.kvm_cpuid' is clear,
__kvm_find_kvm_cpuid_features() fails to find KVM PV feature entry and the
logic which clears KVM_FEATURE_PV_UNHALT after enabling
KVM_X86_DISABLE_EXITS_HLT does not work.

The logic, introduced by the commit ee3a5f9 ("KVM: x86: Do runtime
CPUID update before updating vcpu->arch.cpuid_entries") must stay: the
supplied CPUID data is tweaked by KVM first (__kvm_update_cpuid_runtime())
and checked later (kvm_check_cpuid()) and the actual data
(vcpu->arch.cpuid_*, vcpu->arch.kvm_cpuid, vcpu->arch.xen.cpuid,..) is only
updated on success.

Switch to searching for KVM_SIGNATURE in the supplied CPUID data to
discover KVM PV feature entry instead of using stale 'vcpu->arch.kvm_cpuid'.

While on it, drop pointless "&& (best->eax & (1 << KVM_FEATURE_PV_UNHALT)"
check when clearing KVM_FEATURE_PV_UNHALT bit.

Fixes: ee3a5f9 ("KVM: x86: Do runtime CPUID update before updating vcpu->arch.cpuid_entries")
Reported-and-tested-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240228101837.93642-3-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: vittyvk

arch/x86/kvm/cpuid.c

@@ -224,22 +224,22 @@ 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_vcpu *vcpu,
-					      struct kvm_cpuid_entry2 *entries, int nent)
+static struct kvm_cpuid_entry2 *__kvm_find_kvm_cpuid_features(struct kvm_cpuid_entry2 *entries,
+							      int nent, u32 kvm_cpuid_base)
 {
-	u32 base = vcpu->arch.kvm_cpuid.base;
-
-	if (!base)
-		return NULL;
-
-	return cpuid_entry2_find(entries, nent, base | KVM_CPUID_FEATURES,
+	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)
 {
-	return __kvm_find_kvm_cpuid_features(vcpu, vcpu->arch.cpuid_entries,
-					     vcpu->arch.cpuid_nent);
+	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);
 }
 
 void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
@@ -273,6 +273,7 @@ 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) {
@@ -299,10 +300,12 @@ 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);
 
-	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
-	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
-		(best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
-		best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
+	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);