Re: f_ops flag to speed up compatible ioctls in linux kernel

[Roland Dreier]

    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.

Thanks,
  Roland

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