[PATCH] Use SYSCALL_LL[64] to pass 64-bit value [BZ #20349]

[Chris Metcalf]

On 7/11/2016 3:26 PM, H.J. Lu wrote:
> SYSCALL_LL/SYSCALL_LL64 should be used to pass 64-bit value to system
> calls.

What problem are you trying to solve?

SYSCALL_LL and LO_HI_LONG are different on big-endian systems.  In this
case, LO_HI_LONG is correct, since the kernel API is "unsigned long pos_l,
unsigned long pos_h".  SYSCALL_LL won't do the right thing.

__ALIGNMENT_ARG introduces an extra dummy arguments to be inserted before
a 64-bit value split into two 32-bit registers, if required by the platform.
Since preadv/pwritev explicitly use split arguments to construct a 64-bit
loff_t, __ALIGNMENT_ARG will just add a random inappropriate dummy arg
to an API that doesn't need one.

I reviewed the casting for LO_HI_LONG and it looks OK; I initially was
wondering whether losing the "val >> 31, val" semantics from SYSCALL_LL()
might have been problematic, but it seems like LO_HI_LONG should generate
the same results.

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com