KVM: x86: Wrong emulation on 'xadd X, X'

The emulator does not emulate the xadd instruction correctly if the two
operands are the same.  In this (unlikely) situation the result should be the
sum of X and X (2X) when it is currently X.  The solution is to first perform
writeback to the source, before writing to the destination.  The only
instruction which should be affected is xadd, as the other instructions that
perform writeback to the source use the extended accumlator (e.g., RAX:RDX).
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -4736,17 +4736,17 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		goto done;
 
 writeback:
-	if (!(ctxt->d & NoWrite)) {
-		rc = writeback(ctxt, &ctxt->dst);
-		if (rc != X86EMUL_CONTINUE)
-			goto done;
-	}
 	if (ctxt->d & SrcWrite) {
 		BUG_ON(ctxt->src.type == OP_MEM || ctxt->src.type == OP_MEM_STR);
 		rc = writeback(ctxt, &ctxt->src);
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
 	}
+	if (!(ctxt->d & NoWrite)) {
+		rc = writeback(ctxt, &ctxt->dst);
+		if (rc != X86EMUL_CONTINUE)
+			goto done;
+	}
 
 	/*
 	 * restore dst type in case the decoding will be reused