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From: Russell King <rmk+lkml@arm.linux.org.uk>
To: Scott Lurndal <scott.lurndal@3leafsystems.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>,
	Matthew Wilcox <matthew@wil.cx>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	Ulrich Drepper <drepper@redhat.com>
Subject: Re: [PATCH v2] Add preadv and pwritev system calls.
Date: Fri, 12 Dec 2008 19:07:28 +0000	[thread overview]
Message-ID: <20081212190728.GC5774@flint.arm.linux.org.uk> (raw)
In-Reply-To: <20081212182929.GA9631@pendragon.3leafnetworks.com>

On Fri, Dec 12, 2008 at 10:29:29AM -0800, Scott Lurndal wrote:
> On Fri, Dec 12, 2008 at 05:02:05PM +0100, Gerd Hoffmann wrote:
> > Matthew Wilcox wrote:
> > > On the other hand, NetBSD have approximately 0% market share.
> > > We shouldn't let them lock us into making a bad decision.  Is there
> > > anyone other than NetBSD who has added these syscalls?
> > 
> > Free- and OpenBSD have it too.  For Solaris I've found a feature request
> > only.  Dunno about MacOS/Darwin.  Other un*xes which are important these
> > days?
> > 
> > I'd *really* hate it to have the same system call with different
> > argument ordering on different systems though.  Especially when swapping
> > two integer values, so gcc wouldn't error out on wrong usage.
> 
> I would suggest that from the end-users perspective, the user-mode API
> should be similar to pread/pwrite, e.g:
> 
>     int preadv(fd, iovec, iovec_size, offset)

Yes, and that's easy for glibc to achieve.

What's hard is that the user <-> kernel API firstly has a limited number
of registers available to it for passing arguments without indirection
from user space into kernel space.

Secondly, the user <-> kernel argument register allocation can vary
depending on the ABI version which user space or kernel space is built
for.  On ARM we have two ABIs, one where 64-bit arguments can be placed
in any two consecutive registers, and one where 64-bit arguments must
be placed in an even,odd register pair (not an odd,even pair.)

That leads to the above being:

fd	r0	r0
iovec	r1	r1
vecsz	r2	r2
offset	r3,r4	r4,r5

Notice the different register allocation for the 64-bit offset.

This problem of register-aligned argument placement is not limited
to just ARM, but several other Linux supported architectures.

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

  reply	other threads:[~2008-12-12 19:13 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-12-12 14:00 Gerd Hoffmann
2008-12-12 15:29 ` Matthew Wilcox
2008-12-12 15:48   ` Gerd Hoffmann
2008-12-12 15:51     ` Matthew Wilcox
2008-12-12 16:02       ` Gerd Hoffmann
2008-12-12 17:03         ` Matthew Wilcox
2008-12-12 18:21           ` Alan Cox
2008-12-12 19:02             ` Russell King
2008-12-12 18:29         ` Scott Lurndal
2008-12-12 19:07           ` Russell King [this message]
2008-12-12 19:56             ` Gerd Hoffmann
2008-12-12 19:56               ` Gerd Hoffmann
2008-12-12 20:12               ` Russell King
2008-12-12 20:39                 ` Gerd Hoffmann
2008-12-12 20:39                   ` Gerd Hoffmann
2008-12-14 18:19               ` Pavel Machek
2008-12-15 16:37         ` Jennifer Pioch
2008-12-15 20:43           ` Gerd Hoffmann
2008-12-16  9:57             ` Arnd Bergmann
2008-12-17  1:45               ` [osol-code] " Dan Mick
2008-12-17  1:45                 ` Dan Mick
2008-12-12 19:47   ` Arnd Bergmann
2008-12-12 20:02     ` Gerd Hoffmann
2008-12-14 11:49   ` Heiko Carstens
2008-12-15  4:14   ` Paul Mackerras
2008-12-15  6:20     ` David Miller
2008-12-12 15:40 ` Ralf Baechle
2008-12-12 16:59 ` Russell King
2008-12-13  1:18 ` Michael Kerrisk

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