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SubjectRe: [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@polymtl.ca> wrote:

> * H. Peter Anvin (hpa@zytor.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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