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    SubjectRe: Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support
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    On 12 November 2013 12:57, Masami Hiramatsu
    <masami.hiramatsu.pt@hitachi.com> wrote:
    > (2013/11/12 15:23), Sandeepa Prabhu wrote:
    >>>>> OK, I've ensured that the hw_breakpoint (from perf) can work
    >>>>> with kprobes (from ftrace) at the same address on x86.
    >>>>> So if arm64 already support hw_breakpoint on perf, kprobes should
    >>>>> work with it.
    >>>>
    >>>> Single-stepping on x86 is different to the step behaviour on arm64 afaik. On
    >>>> ARM, we have to manually remove the breakpoint, perform a single-step, then
    >>>> add the breakpoint again. If we re-enable debug exceptions in the kprobe
    >>>> handler, the step will complete early and we'll never step off the
    >>>> breakpoint.
    >>>
    >>> I'm unsure about arm64's debug feature behavior, what does happen when
    >>> it performs a single-step on sw-breakpoint?
    >>>
    >>>> Sandeepa: I think you need to retry Masami's test on the arm64 model, since
    >>>> I'm fairly sure it won't work as expected without some additional code.
    >>>
    >>> OK, anyway, for testing same one, we need to port ftrace first. So the next
    >
    > Sorry for confusion, s/next/fallback is what I meant. Making a kprobe module
    > can be done without ftrace port.
    Yes, got it, all my verification until now are done using sample
    modules only, looking out for perf (or some other mechanism: ptrace?)
    that uses v8 hw breakpoint.
    >
    >>> plan is to make a kprobe module to put a probe (which just printk something)
    >>> on a specific function (e.g. vfs_symlink), and run perf record with
    >>> hw-breakpoint as below
    >>>
    >>> $ perf record -e "mem:0xXXXXXX:k" ln -s /dev/null /tmp/foo
    >>>
    >>> Note that 0xXXXXXX is the address of vfs_symlink.
    >>>
    >>> After that, you can see the message in dmesg and also check the perf result
    >>> with "sudo perf script --dump" (you can find a PERF_RECORD_SAMPLE entry if
    >>> it works)
    >> Thanks for steps, ARM64 ftrace patches are under review on arm mailing
    >> list, I can contact the (linaro) developer implementing ftrace on
    >> what's supported and then figure-out a way to test this concurrency of
    >> kprobes breakpoint and hardware breakpoint.
    >
    > Would you mean this? :)
    > http://www.spinics.net/lists/arm-kernel/msg278477.html
    >
    > Wow, it seems that this also has some works around instruction
    > manipulation (and confusable filenames...)
    I referred to: http://lwn.net/Articles/572323/ which is another
    implementation and on LAKML

    >
    > Thank you,
    >
    > --
    > Masami HIRAMATSU
    > IT Management Research Dept. Linux Technology Center
    > Hitachi, Ltd., Yokohama Research Laboratory
    > E-mail: masami.hiramatsu.pt@hitachi.com
    >
    >


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