x86emul: check for CLFLUSH{,OPT} availability

We can't exclude someone wanting to hide either of the instructions
from guests.

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
@@ -1296,6 +1296,7 @@ static bool vcpu_has(
 #define vcpu_has_hle()         vcpu_has(         7, EBX,  4, ctxt, ops)
 #define vcpu_has_rtm()         vcpu_has(         7, EBX, 11, ctxt, ops)
 #define vcpu_has_smap()        vcpu_has(         7, EBX, 20, ctxt, ops)
+#define vcpu_has_clflushopt()  vcpu_has(         7, EBX, 23, ctxt, ops)
 
 #define vcpu_must_have(feat) \
     generate_exception_if(!vcpu_has_##feat(), EXC_UD)
@@ -5104,6 +5105,10 @@ x86_emulate(
         {
         case 7: /* clflush{,opt} */
             fail_if(modrm_mod == 3);
+            if ( !vex.pfx )
+                vcpu_must_have(clflush);
+            else
+                vcpu_must_have(clflushopt);
             fail_if(ops->wbinvd == NULL);
             if ( (rc = ops->wbinvd(ctxt)) != 0 )
                 goto done;