[PATCH] off_t: fix register pair calculation for 64-bit case

[Arnd Bergmann]

On Monday, June 27, 2016 9:11:51 AM CEST Adhemerval Zanella wrote:
> > 
> >> The whole idea of ILP32 patchset is to be a counterpart for kernel code
> >> that clears top halves of registers unconditionally at now. It means we
> >> cannot pass any 64-bit value in a single register, and that's what
> >> the code under sysdeps/unix/sysv/linux/wordsize-64 does. So I don't
> >> understand how we can use it.
> > 
> > The code in sysdeps/unix/sysv/linux/wordsize-64 seems to be made for
> > the 64-bit syscall API, which is not appropriate here as the kernel
> > port uses the 32-bit syscall API, now basically unchanged.
> > 
> > This is the same as tile64/ilp32 does, but different from x86-64/ilp32
> > (x32).
> 
> I intend to send a patch upstream to consolidate all the fallocate
> implementation to help this very issue.  The idea is to use the same
> pread consolidate idea:
> 
>   1. Each architecture/ABI defines if its a ILP32 (__ASSUME_WORDSIZE64_ILP32)
>      and if off64_t differs in size of off_t (__ASSUME_OFF_DIFF_OFF64).
>      Currently, x32 defines __ASSUME_WORDSIZE64_ILP32 and only MIPS64-n32
>      defines both __ASSUME_WORDSIZE64_ILP32 and __ASSUME_OFF_DIFF_OFF64.
>
>   2. For the default function implementation (without the 64 suffix)
>      the symbol will be built if is 32-bits (__WORDSIZE==64) or
>      if off_t differs in size from off64_t (__ASSUME_OFF_DIFF_OFF64).
> 
>      It means that for architecture that only pass 64-bit off_t this
>      symbol won't be build.
> 
>   3. The 64 variant of the function implementation (with the 64 suffix)
>      will be always build and a weak alias for the non-suffix variant
>      will be created if __WORDSIZE == 64 and if size of off64_t differs
>      from off_t.
> 
>      It means that for architecture that only pass 64-bits off_t 
>      function will be an alias to function64.
> 
> I think with this patch there is no need to more arch-specific implementation.

Doesn't that assume that the kernel interface uses 64-bit registers
to pass off_t?  Yury's patch was specifically for the case where
you use two 32-bit registers (or two lower halves of 64-bit registers
in case of aarch64) but still want 64-bit off_t by default,
i.e. (!defined(__ASSUME_WORDSIZE64_ILP32) && !defined(__ASSUME_OFF_DIFF_OFF64)).

	Arnd