x86emul: use DstEax also for XCHG

Just like said in commit c0bc0adf24 ("x86emul: use DstEax where
possible"): While it avoids just a few instructions, we should
nevertheless make use of generic code as much as possible. Here we can
arrange for that by simply swapping source and destination (as they're
interchangeable).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -118,8 +118,10 @@ static uint8_t opcode_table[256] = {
     DstMem|SrcReg|ModRM|Mov, DstReg|SrcNone|ModRM,
     DstReg|SrcMem16|ModRM|Mov, DstMem|SrcNone|ModRM|Mov,
     /* 0x90 - 0x97 */
-    ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
-    ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
+    DstEax|SrcImplicit, DstEax|SrcImplicit,
+    DstEax|SrcImplicit, DstEax|SrcImplicit,
+    DstEax|SrcImplicit, DstEax|SrcImplicit,
+    DstEax|SrcImplicit, DstEax|SrcImplicit,
     /* 0x98 - 0x9F */
     ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
     ImplicitOps|Mov, ImplicitOps|Mov, ImplicitOps, ImplicitOps,
@@ -2491,16 +2493,14 @@ x86_emulate(
 
     case 0x90: /* nop / xchg %%r8,%%rax */
         if ( !(rex_prefix & 1) )
-            break; /* nop */
+            goto no_writeback; /* nop */
 
     case 0x91 ... 0x97: /* xchg reg,%%rax */
-        src.type = dst.type = OP_REG;
-        src.bytes = dst.bytes = op_bytes;
-        src.reg  = (unsigned long *)&_regs.eax;
-        src.val  = *src.reg;
-        dst.reg  = decode_register(
+        src.type = OP_REG;
+        src.bytes = op_bytes;
+        src.reg  = decode_register(
             (b & 7) | ((rex_prefix & 1) << 3), &_regs, 0);
-        dst.val  = *dst.reg;
+        src.val  = *src.reg;
         goto xchg;
 
     case 0x98: /* cbw/cwde/cdqe */