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

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Message-ID: <20100117111608.35a98ee2@infradead.org>
Date:	Sun, 17 Jan 2010 11:16:08 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	"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, mhiramat@...hat.com
Subject: Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump
 patching without stop_machine

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 ;-)


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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