x86emul: check for SYSENTER/SYSEXIT availability

We can't exclude someone wanting to hide 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
@@ -1306,6 +1306,7 @@ static bool vcpu_has(
 
 #define vcpu_has_fpu()         vcpu_has(         1, EDX,  0, ctxt, ops)
 #define vcpu_has_cx8()         vcpu_has(         1, EDX,  8, ctxt, ops)
+#define vcpu_has_sep()         vcpu_has(         1, EDX, 11, ctxt, ops)
 #define vcpu_has_cmov()        vcpu_has(         1, EDX, 15, ctxt, ops)
 #define vcpu_has_clflush()     vcpu_has(         1, EDX, 19, ctxt, ops)
 #define vcpu_has_mmx()         vcpu_has(         1, EDX, 23, ctxt, ops)
@@ -5132,6 +5133,7 @@ x86_emulate(
         uint64_t msr_content;
         int lm;
 
+        vcpu_must_have(sep);
         generate_exception_if(mode_ring0(), EXC_GP, 0);
         generate_exception_if(!in_protmode(ctxt, ops), EXC_GP, 0);
 
@@ -5179,6 +5181,7 @@ x86_emulate(
     {
         uint64_t msr_content;
 
+        vcpu_must_have(sep);
         generate_exception_if(!mode_ring0(), EXC_GP, 0);
         generate_exception_if(!in_protmode(ctxt, ops), EXC_GP, 0);