KVM: x86: fnstcw and fnstsw may cause spurious exception

Since the operand size of fnstcw and fnstsw is updated during the execution,
the emulation may cause spurious exceptions as it reads the memory beforehand.

Marking these instructions as Mov (since the previous value is ignored) and
DstMem16 to simplify the setting of operand size.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -86,6 +86,7 @@
 #define DstAcc      (OpAcc << DstShift)
 #define DstDI       (OpDI << DstShift)
 #define DstMem64    (OpMem64 << DstShift)
+#define DstMem16    (OpMem16 << DstShift)
 #define DstImmUByte (OpImmUByte << DstShift)
 #define DstDX       (OpDX << DstShift)
 #define DstAccLo    (OpAccLo << DstShift)
@@ -1057,8 +1058,6 @@ static int em_fnstcw(struct x86_emulate_ctxt *ctxt)
 	asm volatile("fnstcw %0": "+m"(fcw));
 	ctxt->ops->put_fpu(ctxt);
 
-	/* force 2 byte destination */
-	ctxt->dst.bytes = 2;
 	ctxt->dst.val = fcw;
 
 	return X86EMUL_CONTINUE;
@@ -1075,8 +1074,6 @@ static int em_fnstsw(struct x86_emulate_ctxt *ctxt)
 	asm volatile("fnstsw %0": "+m"(fsw));
 	ctxt->ops->put_fpu(ctxt);
 
-	/* force 2 byte destination */
-	ctxt->dst.bytes = 2;
 	ctxt->dst.val = fsw;
 
 	return X86EMUL_CONTINUE;
@@ -3863,7 +3860,7 @@ static const struct gprefix pfx_0f_e7 = {
 };
 
 static const struct escape escape_d9 = { {
-	N, N, N, N, N, N, N, I(DstMem, em_fnstcw),
+	N, N, N, N, N, N, N, I(DstMem16 | Mov, em_fnstcw),
 }, {
 	/* 0xC0 - 0xC7 */
 	N, N, N, N, N, N, N, N,
@@ -3905,7 +3902,7 @@ static const struct escape escape_db = { {
 } };
 
 static const struct escape escape_dd = { {
-	N, N, N, N, N, N, N, I(DstMem, em_fnstsw),
+	N, N, N, N, N, N, N, I(DstMem16 | Mov, em_fnstsw),
 }, {
 	/* 0xC0 - 0xC7 */
 	N, N, N, N, N, N, N, N,