From a9262f497a45d66ec353c9267faf4b3bc45b331b Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sat, 16 Mar 2019 11:12:02 +0000
Subject: [PATCH] amd64: rewrite cpu_switch.S fragment to reload tss.rsp0 on
 context switch.

New code avoids jumps.

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D19514
---
 sys/amd64/amd64/cpu_switch.S | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/sys/amd64/amd64/cpu_switch.S b/sys/amd64/amd64/cpu_switch.S
index fa13b0b9d184..684661c7ff46 100644
--- a/sys/amd64/amd64/cpu_switch.S
+++ b/sys/amd64/amd64/cpu_switch.S
@@ -209,14 +209,11 @@ do_kthread:
 done_tss:
 	movq	%r8,PCPU(RSP0)
 	movq	%r8,PCPU(CURPCB)
-	/* Update the TSS_RSP0 pointer for the next interrupt */
-	cmpq	$~0,PCPU(UCR3)
-	je	1f
 	movq	PCPU(PTI_RSP0),%rax
+	cmpq	$~0,PCPU(UCR3)
+	cmove	%r8,%rax
 	movq	%rax,TSS_RSP0(%rdx)
-	jmp	2f
-1:	movq	%r8,TSS_RSP0(%rdx)
-2:	movq	%r12,PCPU(CURTHREAD)		/* into next thread */
+	movq	%r12,PCPU(CURTHREAD)		/* into next thread */
 
 	/* Test if debug registers should be restored. */
 	testl	$PCB_DBREGS,PCB_FLAGS(%r8)