Re: [patch 143/166] preadv/pwritev: Add preadv and pwritev system calls.

Gerd Hoffmann
2009-04-03T08:37:54Z

Linus Torvalds wrote:
>
> On Thu, 2 Apr 2009, akpm@linux-foundation.org wrote:
>> glibc entriely and use a syscall prototype which works without
>> arch-specific wrappers inside the kernel: The offset argument is
>> explicitly split into two 32bit values.
>
> Grr. That may make sense on 32-bit architectures, but makes no sense
> what-so-ever on 64-bit ones.
>
> Why not just say that it's two "unsigned long" arguments, and split it by
> "sizeof unsigned long". And then, on 64-bit architectures, the high bits
> can be ignored. You can even get gcc to optimize it all away by doing
> something like
>
> #define HALF_LONG (BITS_IN_LONG / 2)
>
> offset = (((loff_t)high << HALF_LONG) << HALF_LONG) | low;
>
> and then gcc should automaticaly notice that shifting up that way is just
> a fancy way of saying "0", and ignore the high bits.
>
> How married are people to this crazy 2x "32 bits" model?

The tricky corner case are 32bit compat syscalls on 64bit kernel, where
a noticable number of archs (IIRC at least ppc, mips and s390) can't
pass unaligned 64bit values through the syscall abi. The 32/32 split
was done because of that.

I think using two unsigned longs instead of two u32 should work. And
allows us to handle 128bit file offsets on 64bit. That I'll call
future-proof interface! Nevertheless I'd like to have comments from the
arch maintainers on the idea.

cheers,
Gerd