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


On Mon, Jul 11, 2016 at 1:36 PM, Chris Metcalf <cmetcalf@mellanox.com> wrote:
> 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.
>

/* Provide a macro to pass the off{64}_t argument on p{readv,writev}{64}.  */
#define LO_HI_LONG(val) \
 (long) (val), \
 (long) (((uint64_t) (val)) >> 32)

is wrong for __ASSUME_WORDSIZE64_ILP32 platform.

-- 
H.J.


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