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    SubjectRe: disabling group leader perf_event
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    Hi Ingo,

    On Mon, Sep 6, 2010 at 6:47 PM, Ingo Molnar <mingo@elte.hu> wrote:
    >> The actual language doesn't really matter.
    >
    > There are 3 basic categories:
    >
    >  1- Most (least abstract) specific code: a block of bytecode in the form
    >    of a simplified, executable, kernel-checked x86 machine code block -
    >    this is also the fastest form. [yes, this is actually possible.]
    >
    >  2- Least specific (most abstract) code: A subset/sideset of C - as it's
    >    the most kernel-developer-trustable/debuggable form.
    >
    >  3- Everything else little more than a dot on the spectrum between the
    >    first two points.
    >
    > I lean towards #2 - but #1 looks interesting too. #3 is distinctly
    > uninteresting as it cannot be as fast as #1 and cannot be as convenient
    > as #2.

    It's a question where you want to push the complexity of parsing the
    language and verifying the executed code. I'd image it's easier to
    evolve an ABI if we use an intermediate form ("bytecode") on the
    kernel side. Supporting multiple versions of a C-like language is
    probably going to be painful. You also probably don't want to put
    heavy-weight compiler optimization passes in the kernel so with an
    intermediate form, you can do much of that in user-space.

    I'm guessing this thing is expected to work on all architectures? If
    that's true, I'd forget about JIT'ing for the time being and write an
    interpreter first because it's much easier to port. There are
    techniques in making an interpreter pretty fast too. Google for
    "inlining interpreter" if you're interested.

    As for the intermediate form, you might want to take a look at Dalvik:

    http://www.netmite.com/android/mydroid/dalvik/docs/dalvik-bytecode.html

    and probably ParrotVM bytecode too. The thing to avoid is stack-based
    instructions like in Java bytecode because although it's easy to write
    interpreters for them, it makes JIT'ing harder (which needs to convert
    stack-based representation to register-based) and probably doesn't
    lend itself well to stack-constrained kernel code.

    Pekka
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