Re: [PATCH] Remove the extra jmp

Coywolf Qi Hunt
2004-02-24T10:11:10Z

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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