[RFC PATCH v5 3/6] KVM: x86: Reject disabling of MWAIT interception when not allowed

[Kechen Lu <kechenl@nvidia.com>]

From: Sean Christopherson <seanjc@google.com>

Reject KVM_CAP_X86_DISABLE_EXITS if userspace attempts to disable MWAIT
exits and KVM previously reported (via KVM_CHECK_EXTENSION) that MWAIT is
not allowed in guest, e.g. because it's not supported or the CPU doesn't
have an aways-running APIC timer.

Fixes: 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT
intercepts")

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kechen Lu <kechenl@nvidia.com>
---
 arch/x86/kvm/x86.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9a77b55142c6..60caa3fd40e5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4326,6 +4326,16 @@ static inline bool kvm_can_mwait_in_guest(void)
 		boot_cpu_has(X86_FEATURE_ARAT);
 }
 
+static u64 kvm_get_allowed_disable_exits(void)
+{
+	u64 r = KVM_X86_DISABLE_VALID_EXITS;
+
+	if (!kvm_can_mwait_in_guest())
+		r &= ~KVM_X86_DISABLE_EXITS_MWAIT;
+
+	return r;
+}
+
 static int kvm_ioctl_get_supported_hv_cpuid(struct kvm_vcpu *vcpu,
 					    struct kvm_cpuid2 __user *cpuid_arg)
 {
@@ -4448,10 +4458,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = KVM_CLOCK_VALID_FLAGS;
 		break;
 	case KVM_CAP_X86_DISABLE_EXITS:
-		r |=  KVM_X86_DISABLE_EXITS_HLT | KVM_X86_DISABLE_EXITS_PAUSE |
-		      KVM_X86_DISABLE_EXITS_CSTATE;
-		if(kvm_can_mwait_in_guest())
-			r |= KVM_X86_DISABLE_EXITS_MWAIT;
+		r |= kvm_get_allowed_disable_exits();
 		break;
 	case KVM_CAP_X86_SMM:
 		if (!IS_ENABLED(CONFIG_KVM_SMM))
@@ -6224,15 +6231,14 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		break;
 	case KVM_CAP_X86_DISABLE_EXITS:
 		r = -EINVAL;
-		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
+		if (cap->args[0] & ~kvm_get_allowed_disable_exits())
 			break;
 
 		mutex_lock(&kvm->lock);
 		if (kvm->created_vcpus)
 			goto disable_exits_unlock;
 
-		if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
-			kvm_can_mwait_in_guest())
+		if (cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT)
 			kvm->arch.mwait_in_guest = true;
 		if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
 			kvm->arch.hlt_in_guest = true;
-- 
2.34.1