From 3fd63ddfdf3541faea762143365dbc70c16fa49e Mon Sep 17 00:00:00 2001
From: Andrew Turner <andrew@FreeBSD.org>
Date: Fri, 5 Feb 2021 11:41:17 +0000
Subject: [PATCH] Limit when we call DELAY from KCSAN on amd64

In some cases the DELAY implementation on amd64 can recurse on a spin
mutex in the i8254 early delay code. Detect when this is going to
happen and don't call delay in this case. It is safe to not delay here
with the only issue being KCSAN may not detect data races.

Reviewed by:	kib
Tested by:	arichardson
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D28895
---
 sys/amd64/include/csan.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/sys/amd64/include/csan.h b/sys/amd64/include/csan.h
index c886daa12f69..9daf8275492c 100644
--- a/sys/amd64/include/csan.h
+++ b/sys/amd64/include/csan.h
@@ -31,6 +31,9 @@
  * $FreeBSD$
  */
 
+#include <sys/timetc.h>
+
+#include <machine/clock.h>
 #include <machine/cpufunc.h>
 #include <machine/stack.h>
 #include <machine/vmparam.h>
@@ -64,7 +67,14 @@ kcsan_md_enable_intrs(uint64_t *state)
 static inline void
 kcsan_md_delay(uint64_t us)
 {
-	DELAY(us);
+	/*
+	 * Only call DELAY if not using the early delay code. The i8254
+	 * early delay function may cause us to recurse on a spin lock
+	 * leading to a panic.
+	 */
+	if ((tsc_is_invariant && tsc_freq != 0) ||
+	    timecounter->tc_quality > 0)
+		DELAY(us);
 }
 
 static void