Add a custom implementation of cpu_lock_delay() for x86.

Avoid using DELAY() since it can try to use spin locks on CPUs without
a P-state invariant TSC.  For cpu_lock_delay(), always use the TSC if
it exists (even if it is not P-state invariant) to delay for a
microsecond.  If the TSC does not exist, read from I/O port 0x84 to
delay instead.

PR:		228768
Reported by:	Roger Hammerstein <cheeky.m@live.com>
Reviewed by:	kib
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D17851

1 files changed, 19 insertions, 0 deletions

diff --git a/sys/x86/x86/delay.c b/sys/x86/x86/delay.c
index 230902ddaa10..c767250954da 100644
--- a/sys/x86/x86/delay.c
+++ b/sys/x86/x86/delay.c
@@ -122,3 +122,22 @@ DELAY(int n)
 	init_ops.early_delay(n);
 	TSEXIT();
 }
+
+void
+cpu_lock_delay(void)
+{
+
+	/*
+	 * Use TSC to wait for a usec if present, otherwise fall back
+	 * to reading from port 0x84.  We can't call into timecounters
+	 * for this delay since timecounters might use spin locks.
+	 *
+	 * Note that unlike delay_tc(), this uses the TSC even if it
+	 * is not P-state invariant.  For this function it is ok to
+	 * wait even a few usecs.
+	 */
+	if (tsc_freq != 0)
+		delay_tsc(1);
+	else
+		inb(0x84);
+}