KVM: x86: Emulator mis-decodes VEX instructions on real-mode

Commit 7fe864dc (KVM: x86: Mark VEX-prefix instructions emulation as
unimplemented, 2014-06-02) marked VEX instructions as such in protected
mode.  VEX-prefix instructions are not supported relevant on real-mode
and VM86, but should cause #UD instead of being decoded as LES/LDS.

Fix this behaviour to be consistent with real hardware.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
[Check for mod == 3, rather than 2 or 3. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -4517,8 +4517,7 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 
 	/* vex-prefix instructions are not implemented */
 	if (ctxt->opcode_len == 1 && (ctxt->b == 0xc5 || ctxt->b == 0xc4) &&
-	    (mode == X86EMUL_MODE_PROT64 ||
-	    (mode >= X86EMUL_MODE_PROT16 && (ctxt->modrm & 0x80)))) {
+	    (mode == X86EMUL_MODE_PROT64 || (ctxt->modrm & 0xc0) == 0xc0)) {
 		ctxt->d = NotImpl;
 	}