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    SubjectRe: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)
    Am 19.12.2011 12:58, schrieb Avi Kivity:
    >> I'd suggest the easy hack first, to get things going - we can
    >> then help out with the proper solution.
    > I think you're underestimating the complexity there. LWP wasn't
    > designed for this.
    >

    LWP is highly limited in its ability's to support more than one
    "LWP-Instance" being active for a thread, IOW it is not possible.
    You can't activate LWP from a threads context and simultaneously
    activate lwp-system-wide-profiling in the way you suggested it,
    Ingo. Either do the first xor do the last, because you only have
    one xsave-area/msr/lwpcb that is read by the hardware and only one
    LWP-Buffer that is written by the hw.

    So, if one thread is running LWP, because he wants to
    (selfmonitoring and stuff [like for what lwp was designed]) and a
    su or u would activate this system-wide-monitoring, both would
    frequently interfere with the each other. I don't think you want
    this to be possible at all.

    Frankly, it was already a pain to get LWP running from in-kernel,
    like it is done now. I would expect a much higher pain, if you
    would want to do this with a transparent buffer, that gets passed
    around each scheduling (and this would permanently eliminate the
    "lightweight" in "LWP").

    best regards,
    - Benjamin


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