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

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Message-ID: <4B548562.6030008@redhat.com>
Date:	Mon, 18 Jan 2010 10:59:30 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"H. Peter Anvin" <hpa@...or.com>, rostedt@...dmis.org,
	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, tglx@...utronix.de, andi@...stfloor.org,
	roland@...hat.com, rth@...hat.com
Subject: Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patching
 without stop_machine

Arjan van de Ven wrote:
> On Sun, 17 Jan 2010 13:55:39 -0500
> Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> 
>> * H. Peter Anvin (hpa@...or.com) wrote:
>>> On 01/14/2010 07:32 AM, Steven Rostedt wrote:
>>>>> +
>>>>> +	/* Replacing 1 byte can be done atomically. */
>>>>> +	if (unlikely(len <= 1))
>>>>> +		return text_poke(addr, opcode, len);
>>>>
>>>> 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. :-)

Thank you,

-- 
Masami Hiramatsu

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

e-mail: mhiramat@...hat.com

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