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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 ;-)


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    Arjan van de Ven Intel Open Source Technology Centre
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