Re: [RFC PATCH 0/5] RISC-V: Add dynamic TSO support

[Peter Zijlstra]

On Fri, Nov 24, 2023 at 12:04:09PM +0100, Jonas Oberhauser wrote:

> > I think ARM64 approached this problem by adding the
> > load-acquire/store-release instructions and for TSO based code,
> > translate into those (eg. x86 -> arm64 transpilers).
> 
> 
> Although those instructions have a bit more ordering constraints.
> 
> I have heard rumors that the apple chips also have a register that can be
> set at runtime.

Oh, I thought they made do with the load-acquire/store-release thingies.
But to be fair, I haven't been paying *that* much attention to the apple
stuff.

I did read about how they fudged some of the x86 flags thing.

> And there are some IBM machines that have a setting, but not sure how it is
> controlled.

Cute, I'm assuming this is the Power series (s390 already being TSO)? I
wasn't aware they had this.

> > IIRC Risc-V actually has such instructions as well, so *why* are you
> > doing this?!?!
> 
> 
> Unfortunately, at least last time I checked RISC-V still hadn't gotten such
> instructions.
> What they have is the *semantics* of the instructions, but no actual opcodes
> to encode them.

Well, that sucks..

> I argued for them in the RISC-V memory group, but it was considered to be
> outside the scope of that group.
> 
> Transpiling with sufficient DMB ISH to get the desired ordering is really
> bad for performance.

Ha!, quite dreadful I would imagine.

> That is not to say that linux should support this. Perhaps linux should
> pressure RISC-V into supporting implicit barriers instead.

I'm not sure I count for much in this regard, but yeah, that sounds like
a plan :-)