Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patching without stop_machine

[Masami Hiramatsu <mhiramat@redhat.com>]

H. Peter Anvin wrote:
> On 01/18/2010 08:52 AM, Mathieu Desnoyers wrote:
>>>
>>> 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 additional characteristic of the int3 instruction (compared to the
>> general case of a single-byte instruction) is that, when executed, it
>> will trigger a trap, run a trap handler and return to the original code,
>> typically with iret. This therefore implies that a serializing
>> instruction is executed before returning to the instructions following
>> the modification site when the breakpoint is hit.
>>
>> So I hand out to Intel's expertise the question of whether single-byte
>> instruction modification is safe or not in the general case. I'm just
>> pointing out that I can very well imagine an aggressive superscalar
>> architecture for which pipeline structure would support single-byte int3
>> patching without any problem due to the implied serialization, but would
>> not support the general-case single-byte modification due to its lack of
>> serialization.
>>
> 
> This is utter and complete nonsense.   You seem to think that everything
> is guaranteed to hit the breakpoint, which is obviously false.
> Furthermore, until you have done the serialization, you're not
> guaranteed the *breakpoint* is seen, so you have the same condition.

In that time frame, I guess that the processor sees non-modified
instruction and executes it. Since we'll wait until serializing on
each processor, I think it is OK for int3-bypass method.

(Of course, this can depend on chip, it is possible that there is a chip
 which causes a fault when it has a cache-discarding signal on current-
 instruction decoding slot. That's also why we are asking this method
 is OK for x86 processors.)

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com