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    SubjectRe: [PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text (but move it)
    (2012/05/09 14:53), Masami Hiramatsu wrote:
    > (2012/05/08 22:04), Steven Rostedt wrote:
    >> Note, the big difference with using ftrace optimization and normal
    >> kprobe jump optimization is that the ftrace one can be used on a preempt
    >> kernel. But this code is still under development. I want to get a
    >> solution for the current code (this patch set) now. It would be nice if
    >> it was ready for 3.5.
    > I doubt that we can really do this. If this is possible, I can make
    > jump optimization work with preemptive kernel.

    Now I'm clear that it can be done only with ftrace-nop.
    What I concerned with is out-of-line execution buffer releasing
    timing. but with the ftrace-nop, we don't need to do that.

    Theoretically, that will be done in following steps;

    1. call mcount (w/ push call-site address+5)
    2. mcount calls ftrace handlers
    3. mcount makes pt_regs on the stack
    - on x86-32, regs->cs should be the call-site address+5, since
    regs->sp must be the top of original stack.
    - on x86-64, it is enough that regs->sp points the top of original
    4. mcount store call-site address+5 to %ax
    5. mcount calls new-optprobe handler (which returns call-site address+5)
    6. mcount restores registers
    7. mcount returns to the call-site address+5.

    Between 6 and 7, original opt-probe has to do out-of-line execution,
    but this ftrace-based one doesn't need to do that. In that case,
    we don't need to allocate nor to release out-of-line execution buffer
    for this probe.

    Thank you!

    Masami HIRAMATSU
    Software Platform Research Dept. Linux Technology Center
    Hitachi, Ltd., Yokohama Research Laboratory

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