linux-kernel - Re: [PATCH v2] Add preadv and pwritev system calls.

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Date:	Fri, 12 Dec 2008 16:48:36 +0100
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Ulrich Drepper <drepper@...hat.com>
Subject: Re: [PATCH v2] Add preadv and pwritev system calls.

Matthew Wilcox wrote:
>> +asmlinkage ssize_t sys_preadv(unsigned long fd, const struct iovec __user *vec,
>> +                              unsigned long vlen, loff_t pos)
>> +asmlinkage ssize_t sys_pwritev(unsigned long fd, const struct iovec __user *vec,
>> +                               unsigned long vlen, loff_t pos)
> 
> Are these prototypes required?  MIPS and PARISC will need wrappers to
> fix them if they are.  These two architectures have an ABI which
> requires 64-bit arguments to be passed in aligned pairs of registers,
> but glibc doesn't know that (and given the existence of syscall(3),
> can't do much about it even if it knew), so some of the arguments end up
> in the wrong registers.
>
> Things will go much better if we can prototype these as:
>
> asmlinkage ssize_t sys_preadv(unsigned int fd,
>                               const struct iovec __user *vec,
> 				loff_t pos, unsigned long vlen);
> asmlinkage ssize_t sys_pwritev(unsigned int fd,
>                                const struct iovec __user *vec,
> 				 loff_t pos, unsigned long vlen);

Hmm.  It is the argument ordering used by NetBSD, thats why I used that
too.  It certainly should be the application-visible ordering.  We'll
have glibc between apps and kernel though, so I think we can reorder the
arguments at syscall level if that helps on these archs.  Cc'ing Ulrich
Drepper for comments on that.

> By the way, why did you make 'fd' an unsigned long?  The rest of the
> kernel uses unsigned int.

sys_{readv,writev} have unsigned long too, this is where I got it from.
 Don't know what the reason for this is, it looks a bit odd indeed.

cheers,
  Gerd

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