Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patchingwithout stop_machine

From: H. Peter Anvin
Date: Mon Jan 18 2010 - 11:27:46 EST

On 01/18/2010 07:59 AM, Masami Hiramatsu wrote:
>>>>> This part bothers me. The text_poke just writes over the text
>>>>> directly (using a separate mapping). But if that memory is in the
>>>>> pipeline of another CPU, I think this could cause a GPF.
>>>> Could you clarify why you think that?
>>> Basically, what Steven and I were concerned about in this particular
>>> patch version is the fact that this code took a "shortcut" for
>>> single-byte text modification, thus bypassing the int3-bypass scheme
>>> altogether.
>> single byte instruction updates are likely 100x safer than any scheme
>> of multi-byte instruction scheme that I have seen, other than a full
>> stop_machine().
>> That does not mean it is safe, it just means it's an order of
>> complexity less to analyze ;-)
> Yeah, so in the latest patch, I updated it to use int3 even if
> len == 1. :-)

This really doesn't make much sense to me. The whole basis for the int3
scheme itself is that single-byte updates are atomic, so if single-byte
updates can't work -- and as I stated, we at Intel OTC currently believe
it safe -- then int3 can't work either.

The one thing to watch out for is that unless you force an IPI/IRET
cycle afterwards, you can't know when any particular remote processor
will see the update.


H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.

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