Now that the pmap is more paranoid about some shootdowns (pmap.c rev 1.119),

avoid some TLB flushes by not reloading %cr3 when the value isn't changing.

original diff by and ok mlarkin@

diff --git a/sys/arch/amd64/amd64/locore.S b/sys/arch/amd64/amd64/locore.S
index ecc9eed..a205e68 100644 (file)
--- a/sys/arch/amd64/amd64/locore.S
+++ b/sys/arch/amd64/amd64/locore.S
@@ -1,4 +1,4 @@
-/*     $OpenBSD: locore.S,v 1.108 2018/09/09 22:09:36 guenther Exp $   */
+/*     $OpenBSD: locore.S,v 1.109 2018/09/12 06:12:59 guenther Exp $   */
 /*     $NetBSD: locore.S,v 1.13 2004/03/25 18:33:17 drochner Exp $     */
 
 /*
@@ -445,9 +445,15 @@ restore_saved:
        /* Stack pivot done, setup RETGUARD */
        RETGUARD_SETUP_OFF(cpu_switchto, r11, 6*8)
 
-       movq    PCB_CR3(%r13),%rax
+       /* don't switch cr3 to the same thing it already was */
+       movq    %cr3,%rax
+       cmpq    PCB_CR3(%r13),%rax
+       movq    PCB_CR3(%r13),%rax      /* flags from cmpq unchanged */
+       jz      .Lsame_cr3
+
        movq    %rax,%cr3                       /* %rax used below too */
 
+.Lsame_cr3:
        /*
         * If we switched from a userland thread with a shallow call stack
         * (e.g interrupt->ast->mi_ast->prempt->mi_switch->cpu_switchto)