Date: Mon, 27 Apr 2015 17:59:42 -0700 From: Isaac Dunham <ibid.ag@...il.com> To: musl@...ts.openwall.com Cc: yuri.nunami@...wc.com, sumpei.kawasaki@...wc.com Subject: Re: musl sh2 support 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: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50457 > > 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). > 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 libc.so 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. HTH, Isaac Dunham
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