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Replace EPIA_HACK code with a version that 'just' trashes any return 

address cache.  This seems to be rather more effective!
This seems to be adequate and is more justifyable than the previous hack.
This commit is contained in:
dsl 2009-11-21 11:54:47 +00:00
parent ef19c35a6d
commit 3adafc2801
1 changed files with 21 additions and 32 deletions
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53
sys/arch/i386/stand/lib/realprot.S
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@ -1,4 +1,4 @@
/* $NetBSD: realprot.S,v 1.8 2009/02/16 22:39:30 jmcneill Exp $ */
/* $NetBSD: realprot.S,v 1.9 2009/11/21 11:54:47 dsl Exp $ */
/*-
* Copyright (c) 2003 The NetBSD Foundation, Inc.
@ -184,33 +184,31 @@ ENTRY(real_to_prot)
/*
* 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.
* VIA C3 processors (Eden, Samuel 2) 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.
* Further investication indicates that the caching of return addresses
* is most likely the problem.
*
* 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.
* Previous versions just used some extra call/ret and a few NOPs, these
* only helped a bit, but booting compressed kernels would still fail.
*
* Neither of these individually are effective, but this combination is
* determined experimentally to be sufficient.
* Trashing the return address stack (by doing 'call' without matched 'ret')
* Seems to fix things completely. 1 iteration isn't enough, 16 is plenty.
*/
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
#ifdef EPIA_HACK
push %ecx
push $0x10
pop %ecx
1: call trash_return_cache
loop 1b
pop %ecx
#endif
/*
* Load the segment registers while still in protected mode.
@ -260,10 +258,6 @@ xreal:
jne 1f
pop %bp
#ifdef EPIA_HACK
call epia_nops
#endif
sti
popl %eax
retl
@ -283,15 +277,10 @@ dump_eax_buff:
. = . + 16
#ifdef EPIA_HACK
epia_nops:
.code16
nop
nop
nop
nop
nop
nop
ret
trash_return_cache:
.code32
pop %eax
jmp *%eax
#endif
/* vtophys(void *)