Re: [PATCH] Remove the extra jmp

Brian Gerst
2004-02-24T03:27:21Z

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".

--
Brian Gerst