From bff7f83d3941c12f1c090528a1e669ca21147797 Mon Sep 17 00:00:00 2001
From: Andriy Gapon <avg@FreeBSD.org>
Date: Thu, 7 Nov 2019 21:14:59 +0000
Subject: [PATCH] IPI_TRACE is not really supported on xen

x86 stack_save_td_running() can work safely only if IPI_TRACE is a
non-maskable interrupt.  But at the moment FreeBSD/Xen does not provide
support for the NMI delivery mode.  So, mark the functionality as
unsupported similarly to other platforms without NMI.
Maybe there is a way to provide a Xen-specific working
stack_save_td_running(), but I couldn't figure it out.

MFC after:	3 weeks
Sponsored by:	Panzura
---
 sys/x86/x86/stack_machdep.c | 7 +++++++
 1 file changed, 7 insertions(+)

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
@@ -135,6 +135,13 @@ 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));