Re: Remove sparcv8 support

[Torvald Riegel <triegel@redhat.com>]

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.