Subject: Re: [PATCH v6 13/17] arm64:ilp32: add sys_ilp32.c and a separate
 table (in entry.S) to use it
To: Arnd Bergmann <arnd@arndb.de>, <linux-arm-kernel@lists.infradead.org>
References: <1446507046-24604-1-git-send-email-ynorov@caviumnetworks.com>
 <CA+=Sn1mQ+1HRS_=X-RN92yRXD6Xt6xk-ATiX+u=c3_KYsWgtKg@mail.gmail.com>
 <6987238.kVeqDX1hGc@wuerfel> <4020288.s2KSidZP6C@wuerfel>
CC: <bamvor.zhangjian@huawei.com>, Andrew Pinski <pinskia@gmail.com>,
        "Kapoor, Prasun" <Prasun.Kapoor@caviumnetworks.com>,
        Andreas Schwab <schwab@suse.de>,
        Nathan Lynch <Nathan_Lynch@mentor.com>,
        LKML <linux-kernel@vger.kernel.org>, Alexander Graf <agraf@suse.de>,
        Alexey Klimov <klimov.linux@gmail.com>, <broonie@kernel.org>,
        Yury Norov <ynorov@caviumnetworks.com>,
        Andrew Pinski <apinski@cavium.com>,
        David Daney <ddaney.cavm@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jan Dakinevich <jan.dakinevich@gmail.com>,
        Philipp Tomsich <philipp.tomsich@theobroma-systems.com>,
        Andrey Konovalov <andrey.konovalov@linaro.org>,
        <christoph.muellner@theobroma-systems.com>,
        Mike Frysinger <vapier@gentoo.org>,
        "Joseph S. Myers" <joseph@codesourcery.com>,
        Paul Eggert <eggert@cs.ucla.edu>, Rich Felker <dalias@libc.org>
From: Chris Metcalf <cmetcalf@ezchip.com>
Message-ID: <5648B5E3.3060700@ezchip.com>
Date: Sun, 15 Nov 2015 11:42:11 -0500
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On 11/15/2015 10:18 AM, Arnd Bergmann wrote:
> 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@arndb.de> wrote:
>>>> On Thursday 12 November 2015 14:47:18 Andreas Schwab wrote:
>>>>> Arnd Bergmann <arnd@arndb.de> writes:
>>>>>
>>>>>> On Thursday 12 November 2015 10:44:55 Andreas Schwab wrote:
>>>>>>> Arnd Bergmann <arnd@arndb.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?

I think there are two questions here.  The first is whether glibc will change the
default for _FILE_OFFSET_BITS to be 64.  This has been discussed in the past, e.g.:

https://sourceware.org/ml/libc-alpha/2014-03/msg00351.html

I've added Rich, Paul, Joseph, and Mike to the cc's as they are probably a good
subset of libc-alpha to help comment on these issues.  My sense is that right now,
it wouldn't be possible to add a 32-bit architecture with a non-32-bit default
for _FILE_OFFSET_BITS.  And, obviously, this is why, when I added the tilegx32
APIs to glibc in 2011, I needed to provide _FILE_OFFSET_BITS=32 support.

As to the kernel APIs, certainly tilegx32 only has the stat64 API; I just arranged
that the userspace structures are file-offset-bits-agnostic by using ifdefs to
either put a 64-bit value or a (32-bit-value, 32-bit-pad) in the structure.
See sysdeps/unix/sysv/linux/generic/bits/stat.h for example.  While the
__field64() macro is kind of nasty, it does provide the 32-bit off_t to those
programs that want it without any particular cost elsewhere in the code.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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