On Thu, 21 Feb 2019, Björn Töpel wrote:

> On Thu, 21 Feb 2019 at 17:28, Paul Walmsley <paul@pwsan.com> wrote:
> >
> > On Wed, 20 Feb 2019, Björn Töpel wrote:
> >
> > > Christopher mentioned "per-cpu atomicity" in his Plumbers RISC-V HPC
> > > talk [1]. Typically, this would be for the per-cpu implementation in
> > > Linux, so I had a look at the current implementation.
> > >
> > > I noticed that RISC-V currently uses the generic one. Has there been
> > > any work on moving to a RISC-V specific impl, based on the
> > > A-extensions, similar to what aarch64 does? Would that make sense?
> >
> > Yes doing a RISC-V implementation with the AMO instructions would make
> > sense.  Care to take that on?
> >
> 
> Yeah, I'll take a stab at it! I guess AMO is in general favored over
> the LL/SC path, correct?

Yes.  I think our current plan is to only support cores with the 
A-extension in upstream Linux.  Between LR/SC and atomics, the atomics 
would be preferred.  As Christoph wrote, using an atomic add avoids the 
fail/retry risk inherent in an LR/SC sequence, and is easier to optimize 
in microarchitecture.


- Paul