Re: Remove sparcv8 support

[Andreas Larsson <andreas@gaisler.com>]

On 2016-10-27 12:38, Torvald Riegel wrote:
> On Wed, 2016-10-26 at 16:45 +0200, Andreas Larsson wrote:
>> On 2016-10-25 16:44, Adhemerval Zanella wrote:
>>>
>>>
>>>> On 25 Oct 2016, at 12:34, Andreas Larsson <andreas@gaisler.com> wrote:
>>>>
>>>>> On 2016-10-24 19:42, Adhemerval Zanella wrote:
>>>>>
>>>>>
>>>>>> On 24/10/2016 15:25, Torvald Riegel wrote:
>>>>>>> On Fri, 2016-10-21 at 10:59 +0200, Andreas Larsson wrote:
>>>>>>>> On 2016-10-20 21:47, Adhemerval Zanella wrote:
>>>>>>>> The sparcv8 build is broken since GLIBC 2.23 due the new pthread
>>>>>>>> barrier implementation [1] and since then there is no thread or
>>>>>>>> interest on fixing it (Torvald has suggested some options on
>>>>>>>> 2.23 release thread).  It won't help with both new pthread rdlock
>>>>>>>> and cond implementation, although I would expect that it relies
>>>>>>>> on same atomic primitive that was not present for pthread barrier.
>>>>>>>>
>>>>>>>> AFAIK, recent commercial sparc chips from Oracle all supports
>>>>>>>> sparcv9.  The only somewhat recent sparc chip with just sparcv8
>>>>>>>> support is LEON4, which I really doubt it cares for glibc support.
>>>>>>>
>>>>>>> Hi!
>>>>>>>
>>>>>>> We do care about GLIBC support for many different LEON3 and LEON4
>>>>>>> systems. GLIBC support for sparcv8 is important for us and it is
>>>>>>> important for our customers. Both LEON3 and LEON4 are continuously used
>>>>>>> in new hardware designs.
>>>>>>
>>>>>> If you do care about it, it would be nice if you could (help) maintain
>>>>>> sparcv8 (e.g., regularly testing most recent glibc on sparcv8, at the
>>>>>> very least early during the freeze of each release).  This ensures that
>>>>>> you won't get surprises such as this one, when nobody else is spending
>>>>>> resources on it.
>>>>>>
>>>>>>> We are not always using the latest version of GLIBC (the latest step we
>>>>>>> took was to GLIBC 2.20), so unfortunately we missed this issue. We will
>>>>>>> look into what the extent of the missing support is. Any pointers are
>>>>>>> most welcome.
>>>>>>>
>>>>>>> Do you have a link to the suggested options on the 2.23 release thread?
>>>>>>> I dug around a bit in the archives, but did not find it.
>>>>>>>
>>>>>>> (As a side note, most of the recent LEON3 and LEON4 chips have CAS
>>>>>>> instruction support, but pure sparcv8 support is of course the baseline.)
>>>>>>
>>>>>> Yes, the lack of CAS is the major problem I am aware of.  If the chips
>>>>>> you mention do support CAS, then a patch that adds support for the
>>>>>> CAS-based atomic operations in glibc would fix the barrier problem
>>>>>> (because the generic barrier should just work).  The patch would also
>>>>>> have to add configure bits or whatever would be appropriate so that
>>>>>> glibc can figure out whether it is supposed to be run on a sparcv8 with
>>>>>> or without CAS.
>>>>>>
>>>>>> What about stopping support for plain sparcv8, and keeping to support
>>>>>> sparcv8+CAS provided that we have a (group of) maintainer(s) for the
>>>>>> latter that can tend to the minimal responsibilities of an arch
>>>>>> maintainer and has the time to do that?
>>>>>
>>>>> At least the build for sparcv9-linux-gnu with -mcpu=leon3 finishes,
>>>>> although I am not sure if it correctly runs on leon processors.
>>>>> And I seconded Tovarld's suggestion about stop maintaining plain
>>>>> sparcv8 and set sparcv8+CAS as the base supported sparc32.
>>>>
>>>> I have mixed feelings about this, but it is certainly better than
>>>> throwing out sparcv8 outright.
>>>
>>>>> As pointed out by David Miller, correct support for plain sparcv8
>>>>> could really only be provided with kernel supported.  And when
>>>>> it lands on kernel side, it should work effortlessly with a
>>>>> sparcv8 + cas glibc build.
>>>>
>>>> What do you mean by "work effortlessly with a sparcv8 + cas glibc
>>>> build"?
>>>
>>> Meaning that even if underlying hardware does not support correct CAS,
>>> kernel emulation will provide it and thus a default GLIBC sparc32 build
>>> will work regardless.
>>
>> I am not sure it is as simple as that. Even if the kernel makes sure
>> that an emulated CAS is atomic against another emulated CAS, it would
>> not guarantee atomicity against a plain store instruction on a different
>> CPU, right? For the emulated CAS to work on an SMP system I would think
>> the atomic_store_relaxed and atomic_store_release functions would also
>> need to be handled by the kernel, locking the write out when the CAS is
>> emulated, to keep the interaction linearizable.
>
> Is there still recent sparcv8 hardware that has no native CAS but
> multiple CPU cores?  I think we've used the kernel emulation only on
> non-multi-core systems so far.

There are no LEON3 or LEON4 multiprocessor chips that I am aware of that 
lacks CAS. I cannot rule out that some customer has instantiated such a 
design though.

Best regards,
Andreas Larsson