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

[Adhemerval Zanella]

On 27/06/2016 18:01, Arnd Bergmann wrote:
> 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
> 

So if I understood correctly AArch64/ILP32 will another way to handle 
off_t/off64_t, which indeed the original patch make sense.