diff --git a/sys/x86/x86/stack_machdep.c b/sys/x86/x86/stack_machdep.c
index a7e248832daa..11ccbba770cb 100644
--- a/sys/x86/x86/stack_machdep.c
+++ b/sys/x86/x86/stack_machdep.c
@@ -1,181 +1,188 @@
 /*-
  * Copyright (c) 2015 EMC Corporation
  * Copyright (c) 2005 Antoine Brodin
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
 
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #include "opt_stack.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/kernel.h>
 #include <sys/lock.h>
 #include <sys/mutex.h>
 #include <sys/proc.h>
 #include <sys/stack.h>
 
 #include <machine/pcb.h>
 #include <machine/smp.h>
 
 #include <vm/vm.h>
 #include <vm/vm_param.h>
 #include <vm/pmap.h>
 
 #include <x86/stack.h>
 
 #ifdef __i386__
 #define	PCB_FP(pcb)	((pcb)->pcb_ebp)
 #define	TF_FLAGS(tf)	((tf)->tf_eflags)
 #define	TF_FP(tf)	((tf)->tf_ebp)
 #define	TF_PC(tf)	((tf)->tf_eip)
 
 typedef struct i386_frame *x86_frame_t;
 #else
 #define	PCB_FP(pcb)	((pcb)->pcb_rbp)
 #define	TF_FLAGS(tf)	((tf)->tf_rflags)
 #define	TF_FP(tf)	((tf)->tf_rbp)
 #define	TF_PC(tf)	((tf)->tf_rip)
 
 typedef struct amd64_frame *x86_frame_t;
 #endif
 
 #ifdef STACK
 static struct stack *nmi_stack;
 static volatile struct thread *nmi_pending;
 
 #ifdef SMP
 static struct mtx nmi_lock;
 MTX_SYSINIT(nmi_lock, &nmi_lock, "stack_nmi", MTX_SPIN);
 #endif
 #endif
 
 static void
 stack_capture(struct thread *td, struct stack *st, register_t fp)
 {
 	x86_frame_t frame;
 	vm_offset_t callpc;
 
 	stack_zero(st);
 	frame = (x86_frame_t)fp;
 	while (1) {
 		if ((vm_offset_t)frame < td->td_kstack ||
 		    (vm_offset_t)frame >= td->td_kstack +
 		    td->td_kstack_pages * PAGE_SIZE)
 			break;
 		callpc = frame->f_retaddr;
 		if (!INKERNEL(callpc))
 			break;
 		if (stack_put(st, callpc) == -1)
 			break;
 		if (frame->f_frame <= frame)
 			break;
 		frame = frame->f_frame;
 	}
 }
 
 int
 stack_nmi_handler(struct trapframe *tf)
 {
 
 #ifdef STACK
 	/* Don't consume an NMI that wasn't meant for us. */
 	if (nmi_stack == NULL || curthread != nmi_pending)
 		return (0);
 
 	if (!TRAPF_USERMODE(tf) && (TF_FLAGS(tf) & PSL_I) != 0)
 		stack_capture(curthread, nmi_stack, TF_FP(tf));
 	else
 		/* We were running in usermode or had interrupts disabled. */
 		nmi_stack->depth = 0;
 
 	atomic_store_rel_ptr((long *)&nmi_pending, (long)NULL);
 	return (1);
 #else
 	return (0);
 #endif
 }
 
 void
 stack_save_td(struct stack *st, struct thread *td)
 {
 
 	if (TD_IS_SWAPPED(td))
 		panic("stack_save_td: swapped");
 	if (TD_IS_RUNNING(td))
 		panic("stack_save_td: running");
 
 	stack_capture(td, st, PCB_FP(td->td_pcb));
 }
 
 int
 stack_save_td_running(struct stack *st, struct thread *td)
 {
 
+#ifdef XENHVM
+	/*
+	 * There is no NMI support on Xen, so this code can lead to
+	 * an inter-processor deadlock.
+	 */
+	return (EOPNOTSUPP);
+#endif /* XENHVM */
 #ifdef STACK
 	THREAD_LOCK_ASSERT(td, MA_OWNED);
 	MPASS(TD_IS_RUNNING(td));
 
 	if (td == curthread) {
 		stack_save(st);
 		return (0);
 	}
 
 #ifdef SMP
 	mtx_lock_spin(&nmi_lock);
 
 	nmi_stack = st;
 	nmi_pending = td;
 	ipi_cpu(td->td_oncpu, IPI_TRACE);
 	while ((void *)atomic_load_acq_ptr((long *)&nmi_pending) != NULL)
 		cpu_spinwait();
 	nmi_stack = NULL;
 
 	mtx_unlock_spin(&nmi_lock);
 
 	if (st->depth == 0)
 		return (EAGAIN);
 #else /* !SMP */
 	KASSERT(0, ("curthread isn't running"));
 #endif /* SMP */
 	return (0);
 #else /* !STACK */
 	return (EOPNOTSUPP);
 #endif /* STACK */
 }
 
 void
 stack_save(struct stack *st)
 {
 	register_t fp;
 
 #ifdef __i386__
 	__asm __volatile("movl %%ebp,%0" : "=g" (fp));
 #else
 	__asm __volatile("movq %%rbp,%0" : "=g" (fp));
 #endif
 	stack_capture(curthread, st, fp);
 }