diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index fed732c..8956e02 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -878,8 +878,12 @@ Recognized in debug builds of the hypervisor only.
 Allow use of the Forced Emulation Prefix in HVM guests, to allow emulation of
 arbitrary instructions.
 
-This option is intended for development purposes, and is only available in
-debug builds of the hypervisor.
+This option is intended for development and testing purposes.
+
+*Warning*
+As this feature opens up the instruction emulator to arbitrary
+instruction from an HVM guest, don't use this in production system. No
+security support is provided when this flag is set.
 
 ### hvm\_port80
 > `= <boolean>`
diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 73f79cc..c1e9279 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -59,6 +59,23 @@ config BIGMEM
 
 	  If unsure, say N.
 
+config HVM_FEP
+	bool "HVM Forced Emulation Prefix support" if EXPERT = "y"
+	default DEBUG
+	---help---
+
+	  Compiles in a feature that allows HVM guest to arbitrarily
+	  exercise the instruction emulator.
+
+	  This feature can only be enabled during boot time with
+	  appropriate hypervisor command line option. Please read
+	  hypervisor command line documentation before trying to use
+	  this feature.
+
+	  This is strictly for testing purposes, and not appropriate
+	  for use in production.
+
+	  If unsure, say N.
 endmenu
 
 source "common/Kconfig"
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 22f045e..52d66d4 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -37,6 +37,7 @@
 #include <xen/mem_access.h>
 #include <xen/rangeset.h>
 #include <xen/vm_event.h>
+#include <xen/warning.h>
 #include <asm/shadow.h>
 #include <asm/hap.h>
 #include <asm/current.h>
@@ -97,9 +98,17 @@ boolean_param("hap", opt_hap_enabled);
 
 #ifndef opt_hvm_fep
 /* Permit use of the Forced Emulation Prefix in HVM guests */
-bool_t opt_hvm_fep;
+bool_t __read_mostly opt_hvm_fep;
 boolean_param("hvm_fep", opt_hvm_fep);
 #endif
+static const char __initconst *warning_hvm_fep =
+    "**********************************************\n"
+    "******* WARNING: HVM FORCED EMULATION PREFIX IS AVAILABLE\n"
+    "******* This option is *ONLY* intended to aid testing of Xen.\n"
+    "******* It has implications on the security of the system.\n"
+    "******* Please *DO NOT* use this in production.\n"
+    "**********************************************\n";
+
 
 /* Xen command-line option to enable altp2m */
 static bool_t __initdata opt_altp2m_enabled = 0;
@@ -182,6 +191,9 @@ static int __init hvm_enable(void)
     if ( !opt_altp2m_enabled )
         hvm_funcs.altp2m_supported = 0;
 
+    if ( opt_hvm_fep )
+        warning_add(warning_hvm_fep);
+
     /*
      * Allow direct access to the PC debug ports 0x80 and 0xed (they are
      * often used for I/O delays, but the vmexits simply slow things down).
@@ -3913,6 +3925,7 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
         {
             regs->eip += sizeof(sig);
             regs->eflags &= ~X86_EFLAGS_RF;
+            add_taint(TAINT_HVM_FEP);
         }
     }
 
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index dae7e35..5bf77aa 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -175,6 +175,7 @@ int __init parse_bool(const char *s)
  *  'M' - Machine had a machine check experience.
  *  'B' - System has hit bad_page.
  *  'C' - Console output is synchronous.
+ *  'H' - HVM forced emulation prefix is permitted.
  *
  *      The string is overwritten by the next call to print_taint().
  */
@@ -182,11 +183,12 @@ char *print_tainted(char *str)
 {
     if ( tainted )
     {
-        snprintf(str, TAINT_STRING_MAX_LEN, "Tainted: %c%c%c%c",
+        snprintf(str, TAINT_STRING_MAX_LEN, "Tainted: %c%c%c%c%c",
                  tainted & TAINT_UNSAFE_SMP ? 'S' : ' ',
                  tainted & TAINT_MACHINE_CHECK ? 'M' : ' ',
                  tainted & TAINT_BAD_PAGE ? 'B' : ' ',
-                 tainted & TAINT_SYNC_CONSOLE ? 'C' : ' ');
+                 tainted & TAINT_SYNC_CONSOLE ? 'C' : ' ',
+                 tainted & TAINT_HVM_FEP ? 'H' : ' ');
     }
     else
     {
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index f486ee9..3c8aca8 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -27,7 +27,7 @@
 #include <public/hvm/save.h>
 #include <xen/mm.h>
 
-#ifndef NDEBUG
+#ifdef CONFIG_HVM_FEP
 /* Permit use of the Forced Emulation Prefix in HVM guests */
 extern bool_t opt_hvm_fep;
 #else
diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
index 1c652bb..b1b0fb2 100644
--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -142,6 +142,7 @@ uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c);
 #define TAINT_BAD_PAGE                  (1<<2)
 #define TAINT_SYNC_CONSOLE              (1<<3)
 #define TAINT_ERROR_INJECT              (1<<4)
+#define TAINT_HVM_FEP                   (1<<5)
 extern int tainted;
 #define TAINT_STRING_MAX_LEN            20
 extern char *print_tainted(char *str);
