Date: Wed, 1 Sep 2004 08:32:19 +0100
From: viro@parcelfarce.linux.theplanet.co.uk
To: "Michael S. Tsirkin" <mst@mellanox.co.il>
Cc: discuss@x86-64.org, linux-kernel@vger.kernel.org
Subject: Re: f_ops flag to speed up compatible ioctls in linux kernel
Message-ID: <20040901073218.GQ16297@parcelfarce.linux.theplanet.co.uk>
References: <20040901072245.GF13749@mellanox.co.il>
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On Wed, Sep 01, 2004 at 10:22:45AM +0300, Michael S. Tsirkin wrote:
> Hello!
> Currently, on the x86_64 architecture, its quite tricky to make
> a char device ioctl work for an x86 executables.
> In particular,
>    1. there is a requirement that ioctl number is unique -
>       which is hard to guarantee especially for out of kernel modules

Too bad.

>    2. there's a performance huge overhead for each compat call - there's
>       a hash lookup in a global hash inside a lock_kernel -
>       and I think compat performance *is* important.
> 
> Further, adding a command to the ioctl suddenly requires changing
> two places - registration code and ioctl itself.

So don't add them.  Adding a new ioctl is *NOT* a step to be taken lightly -
we used to be far too accepting in that area and as somebody who'd waded
through the resulting dungpiles over the last months I can tell you that
result is utterly revolting.

Excuse me, but I have zero sympathy to people who complain about obstacles
to dumping more into the same piles - it should be hard.
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