[src/trunk]: src/sys/arch/i386/stand Workaround for issues seen on VIA C3-bas...

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

details:   https://anonhg.NetBSD.org/src/rev/70b7f7d139b8
branches:  trunk
changeset: 581135:70b7f7d139b8
user:      gavan <gavan%NetBSD.org@localhost>
date:      Fri May 27 21:40:09 2005 +0000

description:
Workaround for issues seen on VIA C3-based systems. PR port-i386/26007

Tested on ML5000 board. This may not fix the problem on all variations of the
hardware, but it's likely that variations on the theme will. This workaround
is non-intrusive and should not affect any other CPUs.

diffstat:

 sys/arch/i386/stand/boot/Makefile.boot |   3 +-
 sys/arch/i386/stand/lib/realprot.S     |  45 +++++++++++++++++++++++++++++++++-
 2 files changed, 46 insertions(+), 2 deletions(-)

diffs (91 lines):

diff -r 37f4857bfc23 -r 70b7f7d139b8 sys/arch/i386/stand/boot/Makefile.boot
--- a/sys/arch/i386/stand/boot/Makefile.boot    Fri May 27 15:46:23 2005 +0000
+++ b/sys/arch/i386/stand/boot/Makefile.boot    Fri May 27 21:40:09 2005 +0000
@@ -1,4 +1,4 @@
-# $NetBSD: Makefile.boot,v 1.20 2005/05/05 18:03:04 christos Exp $
+# $NetBSD: Makefile.boot,v 1.21 2005/05/27 21:40:09 gavan Exp $
 
 S=     ${.CURDIR}/../../../../../
 
@@ -67,6 +67,7 @@
 CPPFLAGS+= -DPASS_BIOSGEOM
 CPPFLAGS+= -DPASS_MEMMAP
 #CPPFLAGS+= -DBOOTPASSWD
+CPPFLAGS+= -DEPIA_HACK
 
 # The biosboot code is linked to 'virtual' address of zero and is
 # loaded at physical address 0x10000.
diff -r 37f4857bfc23 -r 70b7f7d139b8 sys/arch/i386/stand/lib/realprot.S
--- a/sys/arch/i386/stand/lib/realprot.S        Fri May 27 15:46:23 2005 +0000
+++ b/sys/arch/i386/stand/lib/realprot.S        Fri May 27 21:40:09 2005 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: realprot.S,v 1.3 2004/02/13 11:36:14 wiz Exp $ */
+/*     $NetBSD: realprot.S,v 1.4 2005/05/27 21:40:09 gavan Exp $       */
 
 /*-
  * Copyright (c) 2003 The NetBSD Foundation, Inc.
@@ -188,7 +188,34 @@
  * Based on the descripton in section 14.5 of the 80386 Programmer's
  * reference book.
  */
+/*
+ * EPIA_HACK
+ *
+ * VIA C3 processors don't seem to correctly switch back to executing
+ * 16 bit code after the switch to real mode and subsequent jump.
+ *
+ * It is speculated that the CPU is prefetching and decoding branch
+ * targets and not invalidating this buffer on the long jump.
+ *
+ * The precise reason for this hack working is still unknown, but
+ * it was determined experimentally on two theories:
+ * 1) Flush the pipeline with NOPs
+ * 2) call/ret after the return from prot_to_real seems to improve matters,
+ *    perhaps by making more work for the branch prediction/prefetch logic.
+ *
+ * Neither of these individually are effective, but this combination is
+ * determined experimentally to be sufficient.
+ */
 ENTRY(prot_to_real)
+#ifdef EPIA_HACK
+       .code32
+       call prot_to_real_main
+       .code16
+       call epia_nops
+       retl
+
+prot_to_real_main:
+#endif
        .code32
        pushl   %eax
 
@@ -240,6 +267,10 @@
        jne     1f
        pop     %bp
 
+#ifdef EPIA_HACK
+       call epia_nops 
+#endif
+
        sti
        popl    %eax
        retl
@@ -258,6 +289,18 @@
 dump_eax_buff:
        . = . + 16
 
+#ifdef EPIA_HACK
+epia_nops:
+       .code16
+       nop
+       nop
+       nop
+       nop
+       nop
+       nop
+       ret
+#endif
+
 /* vtophys(void *)
  * convert boot time 'linear' address to a physical one
  */