Re: [PATCH] Remove the extra jmp

[Coywolf Qi Hunt <coywolf@greatcn.org>]

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
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