KVM: x86: Add a VALID_MASK for the MSR exit reason flags

Add the mask KVM_MSR_EXIT_REASON_VALID_MASK for the MSR exit reason
flags.  This simplifies checks that validate these flags, and makes it
easier to introduce new flags in the future.

No functional change intended.
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Message-Id: <20220921151525.904162-3-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -6230,9 +6230,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		break;
 	case KVM_CAP_X86_USER_SPACE_MSR:
 		r = -EINVAL;
-		if (cap->args[0] & ~(KVM_MSR_EXIT_REASON_INVAL |
-				     KVM_MSR_EXIT_REASON_UNKNOWN |
-				     KVM_MSR_EXIT_REASON_FILTER))
+		if (cap->args[0] & ~KVM_MSR_EXIT_REASON_VALID_MASK)
 			break;
 		kvm->arch.user_space_msr_mask = cap->args[0];
 		r = 0;

include/uapi/linux/kvm.h

@@ -485,6 +485,9 @@ struct kvm_run {
 #define KVM_MSR_EXIT_REASON_INVAL	(1 << 0)
 #define KVM_MSR_EXIT_REASON_UNKNOWN	(1 << 1)
 #define KVM_MSR_EXIT_REASON_FILTER	(1 << 2)
+#define KVM_MSR_EXIT_REASON_VALID_MASK	(KVM_MSR_EXIT_REASON_INVAL   |	\
+					 KVM_MSR_EXIT_REASON_UNKNOWN |	\
+					 KVM_MSR_EXIT_REASON_FILTER)
 			__u32 reason; /* kernel -> user */
 			__u32 index; /* kernel -> user */
 			__u64 data; /* kernel <-> user */