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    SubjectRe: f_ops flag to speed up compatible ioctls in linux kernel
        Chris> You forgot a driver specific filesystem which exposes
    Chris> requests in a file per request type style. Also, there's a
    Chris> simple_transaction type of file which can allow you
    Chris> send/recv data and should eliminate the need for tagging.
    Chris> Example, look at nfsd fs (fs/nfsd/nfsctl.c).

    Thanks for the pointer -- I had a look at this stuff. It seems that
    using the simple_transaction stuff is fairly heavyweight -- if I
    understand correctly, every operation requires userspace to do
    open()-write()-read()-close(), and also uses a page of lowmem. I'm
    not sure if this is the best fit for our requirements with InfiniBand
    drivers: although the user->kernel calls are not in the data path,
    there can still be quite a few of them.

    On the other hand, ioctl() holds the BKL through the whole operation
    so that's suboptimal as well.


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