Re: [PATCH] Remove the extra jmp

[Brian Gerst <bgerst@didntduck.org>]

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