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

[Guo Ren]

On Fri, Nov 24, 2023 at 12:54:30PM +0100, Peter Zijlstra wrote:
> 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.
I could understand the rumor, smart design! Thx for sharing.

> 
> 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 :-)
>