To: Chris Wright <chrisw@osdl.org>
Cc: viro@parcelfarce.linux.theplanet.co.uk, discuss@x86-64.org,
       linux-kernel@vger.kernel.org
Subject: Re: f_ops flag to speed up compatible ioctls in linux kernel
References: <20040901072245.GF13749@mellanox.co.il>
	<20040901073218.GQ16297@parcelfarce.linux.theplanet.co.uk>
	<52zn4a0ysg.fsf@topspin.com> <20040901110225.D1973@build.pdx.osdl.net>
From: Roland Dreier <roland@topspin.com>
Date: Wed, 01 Sep 2004 13:54:33 -0700
In-Reply-To: <20040901110225.D1973@build.pdx.osdl.net> (Chris Wright's
 message of "Wed, 1 Sep 2004 11:02:25 -0700")
Message-ID: <52llftzp4m.fsf@topspin.com>
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    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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