diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index cc874ce..385fe1e 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -284,10 +284,15 @@ static int hvmemul_do_io(
         }
         break;
     }
+    case X86EMUL_UNIMPLEMENTED:
+        ASSERT_UNREACHABLE();
+        /* Fall-through */
     default:
         BUG();
     }
 
+    ASSERT(rc != X86EMUL_UNIMPLEMENTED);
+
     if ( rc != X86EMUL_OKAY )
         return rc;
 
@@ -313,6 +318,9 @@ static int hvmemul_do_io_buffer(
 
     rc = hvmemul_do_io(is_mmio, addr, reps, size, dir, df, 0,
                        (uintptr_t)buffer);
+
+    ASSERT(rc != X86EMUL_UNIMPLEMENTED);
+
     if ( rc == X86EMUL_UNHANDLEABLE && dir == IOREQ_READ )
         memset(buffer, 0xff, size);
 
@@ -405,6 +413,8 @@ static int hvmemul_do_io_addr(
     rc = hvmemul_do_io(is_mmio, addr, &count, size, dir, df, 1,
                        ram_gpa);
 
+    ASSERT(rc != X86EMUL_UNIMPLEMENTED);
+
     if ( rc == X86EMUL_OKAY )
         v->arch.hvm_vcpu.hvm_io.mmio_retry = (count < *reps);
 
@@ -2045,6 +2055,7 @@ int hvm_emulate_one_mmio(unsigned long mfn, unsigned long gla)
     switch ( rc )
     {
     case X86EMUL_UNHANDLEABLE:
+    case X86EMUL_UNIMPLEMENTED:
         hvm_dump_emulation_state(XENLOG_G_WARNING, "MMCFG", &ctxt);
         break;
     case X86EMUL_EXCEPTION:
@@ -2102,6 +2113,7 @@ void hvm_emulate_one_vm_event(enum emul_kind kind, unsigned int trapnr,
          * consistent with X86EMUL_RETRY.
          */
         return;
+    case X86EMUL_UNIMPLEMENTED:
     case X86EMUL_UNHANDLEABLE:
         hvm_dump_emulation_state(XENLOG_G_DEBUG, "Mem event", &ctx);
         hvm_inject_hw_exception(trapnr, errcode);
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 93394c1..43ff5aa 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3723,6 +3723,7 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
     switch ( hvm_emulate_one(&ctxt) )
     {
     case X86EMUL_UNHANDLEABLE:
+    case X86EMUL_UNIMPLEMENTED:
         hvm_inject_hw_exception(TRAP_invalid_op, X86_EVENT_NO_EC);
         break;
     case X86EMUL_EXCEPTION:
diff --git a/xen/arch/x86/hvm/io.c b/xen/arch/x86/hvm/io.c
index bf41954..984db21 100644
--- a/xen/arch/x86/hvm/io.c
+++ b/xen/arch/x86/hvm/io.c
@@ -96,6 +96,7 @@ bool hvm_emulate_one_insn(hvm_emulate_validate_t *validate, const char *descr)
     switch ( rc )
     {
     case X86EMUL_UNHANDLEABLE:
+    case X86EMUL_UNIMPLEMENTED:
         hvm_dump_emulation_state(XENLOG_G_WARNING, descr, &ctxt);
         return false;
 
diff --git a/xen/arch/x86/hvm/vmx/realmode.c b/xen/arch/x86/hvm/vmx/realmode.c
index 12d43ad..cf48139 100644
--- a/xen/arch/x86/hvm/vmx/realmode.c
+++ b/xen/arch/x86/hvm/vmx/realmode.c
@@ -106,7 +106,7 @@ void vmx_realmode_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt)
     if ( hvm_vcpu_io_need_completion(vio) || vio->mmio_retry )
         vio->io_completion = HVMIO_realmode_completion;
 
-    if ( rc == X86EMUL_UNHANDLEABLE )
+    if ( rc == X86EMUL_UNHANDLEABLE || rc == X86EMUL_UNIMPLEMENTED )
     {
         gdprintk(XENLOG_ERR, "Failed to emulate insn.\n");
         goto fail;
diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 8d4f244..2557e21 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -3488,7 +3488,7 @@ static int sh_page_fault(struct vcpu *v,
      * would be a good unshadow hint. If we *do* decide to unshadow-on-fault
      * then it must be 'failable': we cannot require the unshadow to succeed.
      */
-    if ( r == X86EMUL_UNHANDLEABLE )
+    if ( r == X86EMUL_UNHANDLEABLE || r == X86EMUL_UNIMPLEMENTED )
     {
         perfc_incr(shadow_fault_emulate_failed);
 #if SHADOW_OPTIMIZATIONS & SHOPT_FAST_EMULATION
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index c1e2300..f334de9 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -848,7 +848,8 @@ do{ asm volatile (                                                      \
                 stub.func);                                             \
         generate_exception_if(res_.fields.trapnr == EXC_UD, EXC_UD);    \
         domain_crash(current->domain);                                  \
-        goto cannot_emulate;                                            \
+        rc = X86EMUL_UNHANDLEABLE;                                      \
+        goto done;                                                      \
     }                                                                   \
 } while (0)
 #else
@@ -2585,7 +2586,7 @@ x86_decode(
                         d = twobyte_table[0x3a].desc;
                         break;
                     default:
-                        rc = X86EMUL_UNHANDLEABLE;
+                        rc = X86EMUL_UNRECOGNIZED;
                         goto done;
                     }
                 }
@@ -2599,7 +2600,7 @@ x86_decode(
                 }
                 else
                 {
-                    rc = X86EMUL_UNHANDLEABLE;
+                    rc = X86EMUL_UNRECOGNIZED;
                     goto done;
                 }
 
@@ -2879,7 +2880,7 @@ x86_decode(
 
     default:
         ASSERT_UNREACHABLE();
-        return X86EMUL_UNHANDLEABLE;
+        return X86EMUL_UNIMPLEMENTED;
     }
 
     if ( ea.type == OP_MEM )
@@ -4191,7 +4192,7 @@ x86_emulate(
                 break;
             case 4: /* fldenv - TODO */
                 state->fpu_ctrl = true;
-                goto cannot_emulate;
+                goto unimplemented_insn;
             case 5: /* fldcw m2byte */
                 state->fpu_ctrl = true;
                 if ( (rc = ops->read(ea.mem.seg, ea.mem.off, &src.val,
@@ -4202,7 +4203,7 @@ x86_emulate(
                 break;
             case 6: /* fnstenv - TODO */
                 state->fpu_ctrl = true;
-                goto cannot_emulate;
+                goto unimplemented_insn;
             case 7: /* fnstcw m2byte */
                 state->fpu_ctrl = true;
                 emulate_fpu_insn_memdst("fnstcw", dst.val);
@@ -4438,7 +4439,7 @@ x86_emulate(
             case 4: /* frstor - TODO */
             case 6: /* fnsave - TODO */
                 state->fpu_ctrl = true;
-                goto cannot_emulate;
+                goto unimplemented_insn;
             case 7: /* fnstsw m2byte */
                 state->fpu_ctrl = true;
                 emulate_fpu_insn_memdst("fnstsw", dst.val);
@@ -5197,7 +5198,7 @@ x86_emulate(
 #undef _GRP7
 
         default:
-            goto cannot_emulate;
+            goto unimplemented_insn;
         }
         break;
     }
@@ -6195,7 +6196,7 @@ x86_emulate(
                 /* vpsll{w,d} $imm8,{x,y}mm,{x,y}mm */
             break;
         default:
-            goto cannot_emulate;
+            goto unimplemented_insn;
         }
     simd_0f_shift_imm:
         generate_exception_if(ea.type != OP_REG, EXC_UD);
@@ -6243,7 +6244,8 @@ x86_emulate(
         case 6: /* psllq $imm8,mm */
             goto simd_0f_shift_imm;
         }
-        goto cannot_emulate;
+        rc = X86EMUL_UNRECOGNIZED;
+        goto done;
 
     case X86EMUL_OPC_66(0x0f, 0x73):
     case X86EMUL_OPC_VEX_66(0x0f, 0x73):
@@ -6259,7 +6261,8 @@ x86_emulate(
                 /* vpslldq $imm8,{x,y}mm,{x,y}mm */
             goto simd_0f_shift_imm;
         }
-        goto cannot_emulate;
+        rc = X86EMUL_UNRECOGNIZED;
+        goto done;
 
     case X86EMUL_OPC(0x0f, 0x77):        /* emms */
     case X86EMUL_OPC_VEX(0x0f, 0x77):    /* vzero{all,upper} */
@@ -6323,7 +6326,8 @@ x86_emulate(
         case 0: /* extrq $imm8,$imm8,xmm */
             break;
         default:
-            goto cannot_emulate;
+            rc = X86EMUL_UNRECOGNIZED;
+            goto done;
         }
         /* fall through */
     case X86EMUL_OPC_F2(0x0f, 0x78):     /* insertq $imm8,$imm8,xmm,xmm */
@@ -6518,7 +6522,7 @@ x86_emulate(
                 goto done;
             break;
         default:
-            goto cannot_emulate;
+            goto unimplemented_insn;
         }
         break;
 
@@ -6534,7 +6538,8 @@ x86_emulate(
             vcpu_must_have(avx);
             goto stmxcsr;
         }
-        goto cannot_emulate;
+        rc = X86EMUL_UNRECOGNIZED;
+        goto done;
 
     case X86EMUL_OPC_F3(0x0f, 0xae): /* Grp15 */
         fail_if(modrm_mod != 3);
@@ -6777,7 +6782,8 @@ x86_emulate(
             switch ( modrm_reg & 7 )
             {
             default:
-                goto cannot_emulate;
+                rc = X86EMUL_UNRECOGNIZED;
+                goto done;
 
 #ifdef HAVE_GAS_RDRAND
             case 6: /* rdrand */
@@ -7359,7 +7365,8 @@ x86_emulate(
             host_and_vcpu_must_have(bmi1);
             break;
         default:
-            goto cannot_emulate;
+            rc = X86EMUL_UNRECOGNIZED;
+            goto done;
         }
 
         generate_exception_if(vex.l, EXC_UD);
@@ -7670,7 +7677,8 @@ x86_emulate(
             host_and_vcpu_must_have(tbm);
             break;
         default:
-            goto cannot_emulate;
+            rc = X86EMUL_UNRECOGNIZED;
+            goto done;
         }
 
     xop_09_rm_rv:
@@ -7704,7 +7712,8 @@ x86_emulate(
             host_and_vcpu_must_have(tbm);
             goto xop_09_rm_rv;
         }
-        goto cannot_emulate;
+        rc = X86EMUL_UNRECOGNIZED;
+        goto done;
 
     case X86EMUL_OPC_XOP(0a, 0x10): /* bextr imm,r/m,r */
     {
@@ -7736,8 +7745,8 @@ x86_emulate(
     }
 
     default:
-    cannot_emulate:
-        rc = X86EMUL_UNHANDLEABLE;
+    unimplemented_insn:
+        rc = X86EMUL_UNIMPLEMENTED;
         goto done;
     }
 
@@ -7789,7 +7798,8 @@ x86_emulate(
                 if ( (d & DstMask) != DstMem )
                 {
                     ASSERT_UNREACHABLE();
-                    goto cannot_emulate;
+                    rc = X86EMUL_UNHANDLEABLE;
+                    goto done;
                 }
                 break;
             }
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h b/xen/arch/x86/x86_emulate/x86_emulate.h
index 4ddf111..1fb74c0 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -133,6 +133,19 @@ struct x86_emul_fpu_aux {
   * Undefined behavior when used anywhere else.
   */
 #define X86EMUL_DONE           4
+ /*
+  * Current instruction is not implemented by the emulator.
+  * This value should only be returned by the core emulator when a valid
+  * opcode is found but the execution logic for that instruction is missing.
+  * It should NOT be returned by any of the x86_emulate_ops callbacks.
+  */
+#define X86EMUL_UNIMPLEMENTED  5
+ /*
+  * The current instruction's opcode is not valid.
+  * If this error code is returned by a function, an #UD trap should be
+  * raised by the final consumer of it.
+ */
+#define X86EMUL_UNRECOGNIZED   X86EMUL_UNIMPLEMENTED
 
 /* FPU sub-types which may be requested via ->get_fpu(). */
 enum x86_emulate_fpu_type {
