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

[Yury Norov <ynorov at caviumnetworks dot com>]

On Mon, Jun 27, 2016 at 08:00:41PM -0300, Adhemerval Zanella wrote:
> 
> 
> 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.

Yes. You understood correctly. This is how RISC-V works, and this is
default behavior for all new 32-bit ABIs, both native and ILP32.

Is my understanding correct that for AARCH64/ILP32, in glibc terms
we have solid understanding that __ASSUME_OFF_DIFF_OFF64 is disabled;
and__ASSUME_WORDSIZE64_ILP32 depends on kernel wrappers, and under
discussion.

Could you (someone) explain me the difference between
__ASSUME_OFF_DIFF_OFF64 and __OFF_T_MATCHES_OFF64_T?
At first glance they are mutual exclusive, and so we
can drop one of them.

Yury