Re: [PATCH 1/2] futex: mark futex_detect_cmpxchg() as 'noinline'

[Arnd Bergmann]

On Thu, Dec 17, 2020 at 4:32 PM Andreas Larsson <andreas@xxxxxxxxxxx> wrote:
> On 2020-12-16 00:24, Arnd Bergmann wrote:
> > On Tue, Dec 15, 2020 at 8:38 PM Sam Ravnborg <sam@xxxxxxxxxxxx> wrote:
> >> On Tue, Dec 15, 2020 at 12:26:10PM +0100, Arnd Bergmann wrote:
> >>>
> >>> - Disable SMP support for sun4m/sun4d. From the historic git
> >>>    tree, it's unclear how well this ever worked, and very few machines
> >>>    of this class ever existed
> >> Yeah, I have collection of sparc32 machines that I played around with
> >> once. Including one sun4d that I brought from a friendly Linux fellow in
> >> the UK. But somehow I lost interest as this is all very nice machines
> >> but not useful for anything real work.
> >>
> >> I think we would be better served dropping support for sun4m and sun4d
> >> from the kernel.
> >
> > This seems appropriate as well to me.
> >
> >> Last I suggested deleting sun4m/sun4d the argument to keep sun4m was that
> >> QEMU supports sun4m - which is a good argument for sun4m. I dunno what
> >> would be needed to migrate QEMU to LEON, see below.
> >
> > "qemu-system-sparc -M help" shows a "leon3_generic" platform, apparently
> > added in 2013. Do you think that would be sufficient?
>
> We have only use QEMU for LEON3 on limited and simpler system
> setups. For example the Zephyr SPARC arch + LEON3 BSP port we recently
> submitted to the Linux Foundation Zephyr project run their test-suite
> successfully on QEMU+LEON3. We will have to look into the QEMU for LEON
> and Linux situation.
>
> We mainly use TSIM that is our own high fidelity simulator.

I don't think there is a need to have many features supported in qemu,
as long as you have enough RAM, block and network devices (which
are trivial if you have PCI or USB).

> >>> - Mark SMP for LEON as temporarily broken. As I see in the LEON
> >>>    patch set, they have changes to enable compare-and-swap-atomic
> >>>    instructions unconditionally, as all SMP Leons have those and
> >>>    seem to require this support already for other things.
>
> Regarding unconditional compare-and-swap-atomic instructions (CASA) for
> LEON. I tried to get those into mainline under the LEON configuration
> option but did not get OK for that at that time, with the feedback to do
> it dynamically instead. I haven't come around to try to get an updated
> patch doing probing instead into mainline yet. If the thought now is to
> drop support for SMP for sparc32, maybe we can have the CASA
> unconditionally on SMP configured kernels instead. Still we'd like to
> have CASA usage even for non-SMP kernels for LEON systems that
> supports it.

It does make sense to require that a single kernel can work on all
possible hardware. So if we remove sun4m/sun4d support, all that
is left is LEON, and you likely wouldn't need to worry about other
CPUs any more.

However, there is still the question whether a single kernel needs
to work on LEON both with and without CASA. Do you still care
about Linux users on LEON cores that do not support CASA, or is
widespread enough that you just make it unconditional for both
SMP and non-SMP?

> >> LEON on the other hand could have some nice future. They are right now
> >> stuck on an older kernel and someone that was motivated should be able
> >> to get LEON4 running on latest upstream.
> >> We had it working in the past - but is was around the time I lost my
> >> sparc interest and no-one jumped in to move it much more forward.
>
> I would not say that we are stuck on an old kernel. It is rather that
> for our official releases we stay on long term support kernels. We still
> aim for kernels based on master to work on LEON. Right now we do have a
> bit of backlog of things to get into upstream.

Ok, good to hear. There were a couple of old bugs that got found
on mainline that made me wonder whether mainline sparc32 was
used on any hardware at all these days.

I looked at your v4.9 patches and didn't see anything too scary
there, in particular once sparc32 becomes leon-only, you no longer
have to worry about making it work across different CPUs.

> > My best guess from the public information I could find on LEON is that
> > it keeps shifting away from Linux on LEON to other OSs, and to
> > and to Linux on NOEL-V.
>
> On the contrary. Our LEON users shows an increased interest in running
> Linux, now that LEON-based systems gains in number of processors,
> processor performance and memory. We are adding NOEL as a new processor
> line. With NOEL we have a 64-bit architecture. We are not shifting from
> LEON to NOEL. LEON is not going away.

Ok.

> > So even though the CPU itself will likely have a long life ahead of it
> > with LEON5 only a year old, it's unclear how many more updates
> > we'll see to the kernel from the current 4.9 based release.
>
> Our latest release was indeed based on 4.9, but we have no plans to stay
> there forever. We just tend to select long term support kernels for our
> official releases. The next step will be to go to 5.4 if not 5.10.

I hope that you can make it to 5.10 then, as this contains the work
I did for 64-bit time_t, which is required if you have users that want to
run systems after 2038.

> We recently released a new Linux 4.9 toolchain where we stepped up to
> GLIBC 2.31 and GCC 10.2. So we do not consider us stuck at old versions
> of any of the involved software.
>
> In addition, non-LTS users are running other mainline kernel versions
> as well.

FWIW, glibc-2.31 does not have support for 64-bit time_t yet, but I
know there was interest in adding sparc support to the musl libc, which
does support 64-bit time_t.

        Arnd