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    SubjectRe: Can we move device drivers into user-space?
    Hi David,

    > the point that you seem to be missing is that the interfaces between
    > the different areas of the kernel are not stable, they change over
    > time.

    The argument was based on the idea that they would stabilize over
    time. However, I realize this may not be true, which was also touched
    upon in a later reply. The heavy-tailed nature of large changes in
    open-source projects seems to put some hard numbers behind that claim [1].

    > When both sides of the interface are in the kernel, this is
    > not a problem, both sides get changed, but if one side was out of
    > the kernel, then you either can't make the change, or have a flag
    > day change where both sides need to change in lock-step (and
    > downgrading is hard as both sides need to change again)

    Assuming the interfaces changes, this follows naturally, of course.

    > This is completely ignoring the performance and security aspects of
    > userspace components vs kernel components.


    > Ted is explaining the performance aspects well, but let's look at
    > the security aspects as well.
    > It's not just a case of "if something in userspace crashes, it
    > doesn't crash the kerenl", it's also a case that "if you have a
    > userspace component, then the kernel must sanity check the userspace
    > interface to defend against rogue userspace". Doing these checks is
    > not cheap (adding to performance overhead), and may not even be
    > possible (how do you know if the command being sent to the SCSI bus
    > is safe or not?)

    No doubt, an open-ended system has its own set of problems. At any
    given system size, the question is how this balances against a closed
    system. The assumption I made was that as the system grows, the
    balance would shift in favor of an open-ended system. This may not be
    the case at all, as you are saying. It would be nice to be able to see
    this in a quantitative manner if possible.

    Thanks for taking the time to respond.



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