[src/trunk]: src/sys/arch/x86/x86 Add a comment about recent AMD CPUs.

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/9823a3c43c0b
branches:  trunk
changeset: 319272:9823a3c43c0b
user:      maxv <maxv%NetBSD.org@localhost>
date:      Wed May 23 10:21:43 2018 +0000

description:
Add a comment about recent AMD CPUs.

diffstat:

 sys/arch/x86/x86/fpu.c |  8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diffs (29 lines):

diff -r 5dd813a35440 -r 9823a3c43c0b sys/arch/x86/x86/fpu.c
--- a/sys/arch/x86/x86/fpu.c    Wed May 23 10:11:07 2018 +0000
+++ b/sys/arch/x86/x86/fpu.c    Wed May 23 10:21:43 2018 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: fpu.c,v 1.31 2018/05/23 10:00:27 maxv Exp $    */
+/*     $NetBSD: fpu.c,v 1.32 2018/05/23 10:21:43 maxv Exp $    */
 
 /*
  * Copyright (c) 2008 The NetBSD Foundation, Inc.  All
@@ -96,7 +96,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.31 2018/05/23 10:00:27 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.32 2018/05/23 10:21:43 maxv Exp $");
 
 #include "opt_multiprocessor.h"
 
@@ -352,6 +352,10 @@
         * Before that, clear the ES bit in the x87 status word if it is
         * currently set, in order to avoid causing a fault in the
         * upcoming load.
+        *
+        * Newer generations of AMD CPUs have CPUID_Fn80000008_EBX[2],
+        * which indicates that FIP/FDP/FOP are restored (same behavior
+        * as Intel). We're not using it though.
         */
        if (fngetsw() & 0x80)
                fnclex();