Re: [PATCH] Remove the extra jmp
From: Coywolf Qi Hunt
Date: Tue Feb 24 2004 - 05:12:13 EST
Brian Gerst wrote:
Coywolf Qi Hunt wrote:
Jamie Lokier wrote:
Your patch uses two instructions to flush the queue (push+ret) instead
of one (jmp or ljmp). Is that documented as reliable? I can easily
imagine an implementation which decodes one instruction after a mode
change predictably, but not two.
I doubt that it makes a difference - we're setting PG, not changing
the instruction format - but I'd like us to be sure it cannot fail on
things like 386s and 486s, and similar non-Intel chips.
push+ret is encouraged/borrowed/stolen from FreeBSD ;) it should be
reliable. And also, old linux uses ret. Since old linux runs on 386,
it is quite reliable. If you still doubt, we can push before PG.
Hello Anvin,
Please either take the push+ret patch or take the one near jmp patch
enclosed in this email. thanks
Coywolf
------------------------------------------------------------------------
--- head.S.orig 2004-02-18 11:57:16.000000000 +0800
+++ head.S 2004-02-24 11:08:34.000000000 +0800
@@ -117,9 +117,6 @@
movl %eax,%cr0 /* ..and set paging (PG) bit */
jmp 1f /* flush the prefetch-queue */
1:
- movl $1f,%eax
- jmp *%eax /* make sure eip is relocated */
-1:
/* Set up the stack pointer */
lss stack_start,%esp
This won't work, because the indirect jump is what adds PAGE_OFFSET to
%eip (remember, call/jump use relative addressing). Either keep just
the indirect jump, or use "jmp __PAGE_OFFSET+1f".
Any jump works. But I think you did explain very well the reason that
the author carelessly or over carefully left the two jumps there.
Coywolf
--
Coywolf Qi Hunt
Admin of http://GreatCN.org and http://LoveCN.org
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