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    SubjectRe: RFC: [Restatement] KBUS messaging subsystem
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    Hi Tony,

    Your description doesn't really explain what you want to use this
    thing exactly for in userspace.

    On Fri, Jul 29, 2011 at 12:48 AM, Tony Ibbs <tibs@tonyibbs.co.uk> wrote:
    > So why did we write it as a kernel module?
    > ==========================================
    > As implementors, a kernel module makes a lot of sense. Not least
    > because:
    >
    > * It gives us a lot of things for free, including list handling,
    >  reference counting, thread safety and (on larger systems)
    >  multi-processor support, which we would otherwise have to write and
    >  debug ourselves. This also keeps our codebase smaller.

    That's not a reason to put this into the kernel, really.

    > * It helps give us reliability, partly because of the code we're
    >  relying on, partly because of the strictures of working in the
    >  kernel, partly by shielding us from userspace.

    So now instead of crashing in userspace, we crash the kernel? This
    seems like a bogus argument as well.

    > * It reduces message copying (we have userspace to kernel back to
    >  userspace, as opposed to a userspace daemon communicating with
    >  clients via sockets)

    Now this sounds like a real reason but you'd have to explain why you
    can't reuse existing zero-copy mechanisms like splice() and tee().

    > * It makes it simple for us to tell when a message recipient has "gone
    >  away", as the kernel will call our "release" callback for us.

    Again, sounds like a reasonable technical requirement but doesn't
    really justify putting all this code into the kernel.

    > * It allows us to provide the functionality on systems without
    >  requiring anything much beyond /dev and maybe /proc in userspace.

    Why is this important?

    Pekka
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