linux-kernel - Re: [PATCH v6 13/17] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

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Date:	Sun, 15 Nov 2015 16:18 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	bamvor.zhangjian@...wei.com, Andrew Pinski <pinskia@...il.com>,
	"Kapoor, Prasun" <Prasun.Kapoor@...iumnetworks.com>,
	Andreas Schwab <schwab@...e.de>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Graf <agraf@...e.de>,
	Alexey Klimov <klimov.linux@...il.com>, broonie@...nel.org,
	Yury Norov <ynorov@...iumnetworks.com>,
	Andrew Pinski <apinski@...ium.com>,
	David Daney <ddaney.cavm@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Jan Dakinevich <jan.dakinevich@...il.com>,
	Philipp Tomsich <philipp.tomsich@...obroma-systems.com>,
	Andrey Konovalov <andrey.konovalov@...aro.org>,
	christoph.muellner@...obroma-systems.com,
	Chris Metcalf <cmetcalf@...hip.com>
Subject: Re: [PATCH v6 13/17] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

On Friday 13 November 2015 17:10:44 Arnd Bergmann wrote:
> On Friday 13 November 2015 07:38:49 Andrew Pinski wrote:
> > On Fri, Nov 13, 2015 at 7:34 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > > On Thursday 12 November 2015 14:47:18 Andreas Schwab wrote:
> > >> Arnd Bergmann <arnd@...db.de> writes:
> > >>
> > >> > On Thursday 12 November 2015 10:44:55 Andreas Schwab wrote:
> > >> >> Arnd Bergmann <arnd@...db.de> writes:
> > >> >>
> > >> >> > What do you mean with 32-bit off_t?
> > >> >>
> > >> >> An ABI with 32-bit off_t, ie. all currently implemented 32-bit ABIs.
> > >> >>
> > >> >> > Do you mean that glibc emulates a 32-bit off_t on top of the 64-bit
> > >> >> > __kernel_loff_t?
> > >> >>
> > >> >> Glibc is bridging the user-space ABI to the kernel ABI.
> > >> >
> > >> > Ok, but why?
> > >>
> > >> That's how the ABI is defined right now.  I didn't make that up.
> > >
> > > Ok, I guess it will remain a mystery then.
> > 
> > The biggest question is here is how much compatibility do we want with
> > other 32bit ABIs?
> > Do we want off_t to be 32bit or 64bit?
> 
> I would much prefer off_t to be defined as __kernel_loff_t unconditionally,
> with no support for _FILE_OFFSET_BITS == 32. This is at least what I had
> in mind when I wrote the asm-generic/unistd.h header.
> 
> We should probably find out what happened for the other glibc ports that
> were implemented for the architectures using this. It's possible that
> there was a good reason for supporting _FILE_OFFSET_BITS == 32 at the
> time, but I can't think of one and maybe it is one that is no longer
> valid.
> 
> Do you know what x86/x32 does for off_t? Do they also implement both
> _FILE_OFFSET_BITS == 32 and _FILE_OFFSET_BITS == 64 on top of the
> 64-bit __kernel_off_t?

I just did a little bit of digging through glibc history and found that
Chris Metcalf added the files that are now in
sysdeps/unix/sysv/linux/generic/wordsize-32/ and that provide the
implementation for 32-bit off_t in glibc on top of the 64-bit
__kernel_off_t.

Chris, do you remember what led to that? Do you think we still need
to have 32-bit off_t on all new architectures, or could we move
on to making 64-bit off_t the default when adding a port?

	Arnd
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