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Date: Mon, 27 Apr 2015 21:08:39 -0400
From: Rich Felker <>
To: Isaac Dunham <>
Subject: Re: musl sh2 support

On Mon, Apr 27, 2015 at 05:59:42PM -0700, Isaac Dunham wrote:
> On Mon, Apr 27, 2015 at 05:36:03PM -0400, Rich Felker wrote:
> > Recently nsz and I have been looking at the state of the sh port and
> > noticed that the gusa soft atomics, which Bobby Bingham (original port
> > author) and I assumed would be sufficient for anything pre-sh4a,
> > actually don't work on pre-sh3 targets. This is explained on the GCC
> > bug-tracker threads here:
> > 
> >
> > 
> > but the TL;DR is that gusa works by setting an invalid stack pointer
> > as a sentinel to the kernel whereas sh1/sh2 exception-handling
> > requires a valid stack pointer. This issue may also affect __unmapself
> > which runs momentarily (roughly 1-2 cycles in userspace) without a
> > valid stack pointer. For non-SMP configurations I suspect it should
> > suffice for __unmapself to just set the stack pointer to point at some
> > global data for the kernel to use momentarily during exceptions.
> > Alternatively the first thread to call __unmapself could transform
> > into a reaper that never exits but unmaps future detached exiting
> > threads; this could even be a decent default C-only implementation of
> > __unmapself for archs/ABIs that can't handle threads unmapping their
> > own stacks.
> Heads up: Rob Landley's current work project involves bringing up the
> software for a new sh2-compatible chip, the J2 (with BSD-licensed VHDL).
> The latest post on his blog refers to SMP support being "nearly" ready
> (could be done by now, or might not).

Yes. I actually CC'd a couple of the people working on this. I don't
know all the details of their project though.

> > In addition to the question of what to do with atomics, there's a
> > question of whether we need full runtime selection for the atomic
> > method at all. I've been told (but I'm not clear whether it's right)
> > that sh1/sh2(/sh2a?) have a different kernel syscall ABI, and since
> > they're nommu, it wouldn't be possible (or at least not efficiently)
> > to run normal dynamic-linked ELF binaries (where syscall ABI wouldn't
> > matter as long as you have the right installed on the system
> > you're running on) for sh3+ on sh1/2. So it might make sense to treat
> > sh1/sh2 as a separate arch for musl's purposes. But if this arch will
> > possibly have SMP implementations (e.g. running on sh4a or new tech)
> > then soft-tcb atomics will not suffice and it might need its own
> > method of runtime-atomic-selection to get a working atomic cas.
> If the J2 gets done, you will have smp and sh2.

In that case we need to figure out how to make atomics work in a way
that's SMP-compatible. I'm a bit concerned from some things Rob said
that it might not be possible without a machine-wide lock with the
current design...


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