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Re: [PATCH 1/2] MIPS: Loongson, add sync before target of branch between

To: 徐成华 <xuchenghua@loongson.cn>
Subject: Re: [PATCH 1/2] MIPS: Loongson, add sync before target of branch between llsc
From: Paul Burton <paul.burton@mips.com>
Date: Fri, 11 Jan 2019 19:00:50 +0000
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Cc: Yunqiang Su <ysu@wavecomp.com>, Paul Burton <pburton@wavecomp.com>, "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>, "chenhc@lemote.com" <chenhc@lemote.com>, "zhangfx@lemote.com" <zhangfx@lemote.com>, "wuzhangjin@gmail.com" <wuzhangjin@gmail.com>, "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>, 黄沛 <huangpei@loongson.cn>
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Thread-topic: [PATCH 1/2] MIPS: Loongson, add sync before target of branch between llsc
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Hello,

On Fri, Jan 11, 2019 at 08:40:49PM +0800, 徐成华 wrote:
> For Loongson 3A1000 and 3A3000, when a memory access instruction
> (load, store, or prefetch)'s executing occurs between the execution of
> LL and SC, the success or failure of SC is not predictable.  Although
> programmer would not insert memory access instructions between LL and
> SC, the memory instructions before LL in program-order, may
> dynamically executed between the execution of LL/SC, so a memory
> fence(SYNC) is needed before LL/LLD to avoid this situation.
> 
> Since 3A3000, we improved our hardware design to handle this case.
> But we later deduce a rarely circumstance that some speculatively
> executed memory instructions due to branch misprediction between LL/SC
> still fall into the above case, so a memory fence(SYNC) at
> branch-target(if its target is not between LL/SC) is needed for 3A1000
> and 3A3000.

Thank you - that description is really helpful.

I have a few follow-up questions if you don't mind:

 1) Is it correct to say that the only consequence of the bug is that an
    SC might fail when it ought to have succeeded?

 2) Does that mean placing a sync before the LL is purely a performance
    optimization? ie. if we don't have the sync & the SC fails then
    we'll retry the LL/SC anyway, and this time not have the reordered
    instruction from before the LL to cause a problem.

 3) In the speculative execution case would it also work to place a sync
    before the branch instruction, instead of at the branch target? In
    some cases this might be nicer since the workaround would be
    contained within the LL/SC loop, but I guess it could potentially
    add more overhead if the branch is conditional & not taken.

 4) When we talk about branches here, is it really just branch
    instructions that are affected or will the CPU speculate past jump
    instructions too?

I just want to be sure that we work around this properly, and document
it in the kernel so that it's clear to developers why the workaround
exists & how to avoid introducing bugs for these CPUs in future.

> Our processor is continually evolving and we aim to to remove all
> these workaround-SYNCs around LL/SC for new-come processor. 

I'm very glad to hear that :)

I hope one day I can get my hands on a nice Loongson laptop to test
with.

Thanks,
    Paul
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