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Date: Mon, 3 Oct 2016 21:18:03 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Alan Somers <asomers@freebsd.org>
Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>
Subject: Re: Proper way to add vendor-specific syscalls?
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On Mon, Oct 03, 2016 at 11:48:39AM -0600, Alan Somers wrote:
> What's the proper way to add a vendor-specific syscall? The comments
> in kern/syscalls.master suggest that they should be put in the range
> from 151-180, but most of that range is actually occupied.
I think that the 'vendors' there means vendors of other BSD-derived
systems and not e.g. an appliance vendors.

> Only five
> nosys slots are available.  If I add syscalls to the end of the list,
> they'll likely collide with future standard syscalls.  Should I just
> added ~100 nosys syscalls to the end of the list, and put my custom
> syscalls afterwards?  Is there any penalty to lengthening the list?

Each nosys syscall increases the size of the default ABI syscalls table.
On amd64 sizeof(struct sysent) == 48, adding 100 nosys entries would
waste a page and some more.

How many syscalls do you need ?  You probably do not allow random modules
loaded and implementing syscalls, so could you use e.g. range 210-219 ?

If you look further, you will see lot of holes like 258-271, 281-288
etc.  Current practice is to allocate new syscalls at the end of the
table, which leaves there holes unused with high chance of never being
collected.