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Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

On 01/12/2016 01:40 PM, Peter Zijlstra wrote:

It is selectable only for MIPS R2 but not MIPS R6. The reason is - most of
MIPS R2 CPUs have short pipeline and that SYNC is just waste of CPU
resource, especially taking into account that "lightweight syncs" are
converted to a heavy "SYNC 0" in many of that CPUs. However the latest
MIPS/Imagination CPU have a pipeline long enough to hit a problem - absence
of SYNC at LL/SC inside atomics, barriers etc.
What ?! Are you saying that because R2 has short pipelines its unlikely
to hit the reordering issues and we can omit barriers?

It was my guess to explain - why barriers was not included originally. You can check with Ralf, he knows more about that time MIPS Linux code.

I bother with this more than 2 years and I just try to solve that issue - in recent CPUs the load after LL/SC synchronization instruction loop can get ahead of SC for sure, it was tested.

And reading the MIPS64 v6.04 instruction set manual, I think 0x11/0x12
are_NOT_  transitive and therefore cannot be used to implement the
smp_mb__{before,after} stuff.

That is, in MIPS speak, those SYNC types are Ordering Barriers, not
Completion Barriers.
Please see above, point 2.
That did not in fact enlighten things. Are they transitive/multi-copy
atomic or not?

Peter Zijlstra recently wrote: "In particular we're very much all 'confused' about the various notions of transitivity". I am actually confused too and need some examples here.

(and here Will will go into great detail on the differences between the
two and make our collective brains explode :-)

That is, currently all architectures -- with exception of PPC -- have
RCsc locks, but using these non-transitive things will get you RCpc

So yes, MIPS can go RCpc for its locks and share the burden of pain with
PPC, but that needs to be a very concious decision.
I don't understand that - I tried hard but I can't find any word like
"RCsc", "RCpc" in Documents/ directory. Web search goes nowhere, of course.
From: lkml.kernel.org/r/20150828153921.GF19282@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Yes, the difference between RCpc and RCsc is in the meaning of RELEASE +
ACQUIRE. With RCsc that implies a full memory barrier, with RCpc it does

MIPS Arch starting from R2 requires that. If some CPU can't, it should execute a full "SYNC 0" instead, which is a full memory barrier.

Currently PowerPC is the only arch that (can, and) does RCpc and gives a
weaker RELEASE + ACQUIRE. Only the CPU who did the ACQUIRE is guaranteed
to see the stores of the CPU which did the RELEASE in order.

Yes, it was a goal for SYNC_ACQUIRE and SYNC_RELEASE.


- "Full memory barrier" on MIPS means - full barrier for any device in coherent domain. In MIPS Tech/Imagination Tech MIPS-based CPU it is "for any device connected to CM or IOCU + directly connected memory".

- It is not applied to instruction fetch. However, I-Cache flushes and SYNCI are consistent with that. There is also hazard barrier instructions to clear CPU pipeline to some extent - to help with this limitation.

I don't think that these caveats prevent a correct Acquire/Release semantic.

- Leonid.

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