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

[Guo Ren]

On Fri, Nov 24, 2023 at 11:15:19AM +0100, Peter Zijlstra wrote:
> On Fri, Nov 24, 2023 at 08:21:37AM +0100, Christoph Muellner wrote:
> > From: Christoph Müllner <christoph.muellner@xxxxxxxx>
> > 
> > The upcoming RISC-V Ssdtso specification introduces a bit in the senvcfg
> > CSR to switch the memory consistency model at run-time from RVWMO to TSO
> > (and back). The active consistency model can therefore be switched on a
> > per-hart base and managed by the kernel on a per-process/thread base.
> 
> You guys, computers are hartless, nobody told ya?
> 
> > This patch implements basic Ssdtso support and adds a prctl API on top
> > so that user-space processes can switch to a stronger memory consistency
> > model (than the kernel was written for) at run-time.
> > 
> > I am not sure if other architectures support switching the memory
> > consistency model at run-time, but designing the prctl API in an
> > arch-independent way allows reusing it in the future.
> 
> IIRC some Sparc chips could do this, but I don't think anybody ever
> exposed this to userspace (or used it much).
> 
> IA64 had planned to do this, except they messed it up and did it the
> wrong way around (strong first and then relax it later), which lead to
> the discovery that all existing software broke (d'uh).
> 
> 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).
Keeping global TSO order is easier and faster than mixing
acquire/release and regular load/store. That means when ssdtso is
enabled, the transpiler's load-acquire/store-release becomes regular
load/store. Some micro-arch hardwares could speed up the performance.

Of course, you may say powerful machines could smooth out the difference
between ssdtso & load-acquire/store-release, but that's not real life.
Adding ssdtso is a flexible way to gain more choices on the cost of chip
design.

> 
> IIRC Risc-V actually has such instructions as well, so *why* are you
> doing this?!?!
>