KVM: x86: Clarify the kvm_run.emulation_failure structure layout

Until more flags for kvm_run.emulation_failure flags are defined, it
is undetermined whether new payload elements corresponding to those
flags will be additive or alternative. As a hint to userspace that an
alternative is possible, wrap the current payload elements in a union.

Suggested-by: Sean Christopherson <[email protected]>
Signed-off-by: David Edmondson <[email protected]>
Reviewed-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: dme

include/uapi/linux/kvm.h

@@ -403,8 +403,12 @@ struct kvm_run {
 			__u32 suberror;
 			__u32 ndata;
 			__u64 flags;
-			__u8  insn_size;
-			__u8  insn_bytes[15];
+			union {
+				struct {
+					__u8  insn_size;
+					__u8  insn_bytes[15];
+				};
+			};
 		} emulation_failure;
 		/* KVM_EXIT_OSI */
 		struct {