linux-kernel - Re: [patch 06/11] syslets: core, documentation

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Message-ID: <20070214103655.GB4241@flint.arm.linux.org.uk>
Date:	Wed, 14 Feb 2007 10:36:55 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 06/11] syslets: core, documentation

On Tue, Feb 13, 2007 at 03:20:42PM +0100, Ingo Molnar wrote:
> +Arguments to the system call are implemented via pointers to arguments.
> +This not only increases the flexibility of syslet atoms (multiple syslets
> +can share the same variable for example), but is also an optimization:
> +copy_uatom() will only fetch syscall parameters up until the point it
> +meets the first NULL pointer. 50% of all syscalls have 2 or less
> +parameters (and 90% of all syscalls have 4 or less parameters).
> +
> + [ Note: since the argument array is at the end of the atom, and the
> +   kernel will not touch any argument beyond the final NULL one, atoms
> +   might be packed more tightly. (the only special case exception to
> +   this rule would be SKIP_TO_NEXT_ON_STOP atoms, where the kernel will
> +   jump a full syslet_uatom number of bytes.) ]

What if you need to increase the number of arguments passed to a system
call later?  That would be an API change since the size of syslet_uatom
would change?

Also, what if you have an ABI such that:

sys_foo(int fd, long long a)

where:
 arg[0] <= fd
 arg[1] <= unused
 arg[2] <= low 32-bits a
 arg[3] <= high 32-bits a

it seems you need to point arg[1] to some valid but dummy variable.

How do you propose syslet users know about these kinds of ABI issues
(including the endian-ness of 64-bit arguments) ?

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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