[External] Re: [PATCH v2 0/4] riscv: Add Zalasr ISA extension support

[Xu Lu]

Hi Guo Ren,

Thanks for your advice.

On Fri, Sep 19, 2025 at 12:48 AM Guo Ren <guoren at kernel.org> wrote:
>
> On Wed, Sep 17, 2025 at 12:01:34AM -0400, Guo Ren wrote:
> > On Tue, Sep 02, 2025 at 06:59:15PM +0200, Andrea Parri wrote:
> > > > Xu Lu (4):
> > > >   riscv: add ISA extension parsing for Zalasr
> > > >   dt-bindings: riscv: Add Zalasr ISA extension description
> > > >   riscv: Instroduce Zalasr instructions
> > > >   riscv: Use Zalasr for smp_load_acquire/smp_store_release
> > >
> > > Informally put, our (Linux) memory consistency model specifies that any
> > > sequence
> > >
> > >   spin_unlock(s);
> > >   spin_lock(t);
> > >
> > > behaves "as it provides at least FENCE.TSO ordering between operations
> > > which precede the UNLOCK+LOCK sequence and operations which follow the
> > > sequence".  Unless I missing something, the patch set in question breaks
> > > such ordering property (on RISC-V): for example, a "release" annotation,
> > > .RL (as in spin_unlock() -> smp_store_release(), after patch #4) paired
> > > with an "acquire" fence, FENCE R,RW (as could be found in spin_lock() ->
> > > atomic_try_cmpxchg_acquire()) do not provide the specified property.
> > >
> > > I _think some solutions to the issue above include:
> > >
> > >  a) make sure an .RL annotation is always paired with an .AQ annotation
> > >     and viceversa an .AQ annotation is paired with an .RL annotation
> > >     (this approach matches the current arm64 approach/implementation);
> > >
> > >  b) on the opposite direction, always pair FENCE R,RW (or occasionally
> > >     FENCE R,R) with FENCE RW,W (this matches the current approach/the
> > >     current implementation within riscv);
> > >
> > >  c) mix the previous two solutions (resp., annotations and fences), but
> > >     make sure to "upgrade" any releases to provide (insert) a FENCE.TSO.
> > I prefer option c) at first, it has fewer modification and influence.
> Another reason is that store-release-to-load-acquire would give out a
> FENCE rw, rw according to RVWMO PPO 7th rule instead of FENCE.TSO, which
> is stricter than the Linux requirement you've mentioned.

The existing implementation of spin_unlock, when followed by
spin_lock, is equal to 'FENCE rw, rw' for operations before
spin_unlock
 and after spin_lock:

spin_unlock:
    fence rw, w
    sd
spin_lock:
    amocas
    fence r, rw

The store-release semantics in spin_unlock, is used to ensure that
when the other cores can watch the store, they must also watch the
operations before the store, which is a more common case than calling
spin_unlock immediately followed by spin_lock on the same core. And
the existing implementation 'fence rw, w' 'fence r, rw' is stricter
than '.aq' '.rl'. That is why we want to modify it.

I have reimplemented the code and it now looks like the attached text
file. I will send the patch out later.

Best Regards.

Xu Lu

>
> >
> > asm volatile(ALTERNATIVE("fence rw, w;\t\nsb %0, 0(%1)\t\n",  \
> > -                       SB_RL(%0, %1) "\t\nnop\t\n",          \
> > +                       SB_RL(%0, %1) "\t\n fence.tso;\t\n",  \
> >                         0, RISCV_ISA_EXT_ZALASR, 1)           \
> >                         : : "r" (v), "r" (p) : "memory");     \
> >
> > I didn't object option a), and I think it could be done in the future.
> > Acquire Zalasr extension step by step.
> >
> > >
> > > (a) would align RISC-V and ARM64 (which is a good thing IMO), though it
> > > is probably the most invasive approach among the three approaches above
> > > (requiring certain changes to arch/riscv/include/asm/{cmpxchg,atomic}.h,
> > > which are already relatively messy due to the various ZABHA plus ZACAS
> > > switches).  Overall, I'm not too exited at the idea of reviewing any of
> > > those changes, but if the community opts for it, I'll almost definitely
> > > take a closer look with due calm.  ;-)
> > >
> > >   Andrea
> > >
> > > _______________________________________________
> > > linux-riscv mailing list
> > > linux-riscv at lists.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/linux-riscv
> > >
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
> >
-------------- next part --------------
|----------------------------------------------------------------------------------------------|
|    |    arch_xchg_release       |     arch_cmpxchg_release     |     __smp_store_release     |
|    |-----------------------------------------------------------------------------------------|
|    | zabha      | !zabha        | zabha+zacas | !(zabha+zacas) | zalasr        | !zalasr     |
| rl |-----------------------------------------------------------------------------------------|
|    |            | (fence rw, w) |             | (fence rw, w)  |               | fence rw, w |
|    | amoswap.rl | lr.w          | amocas.rl   | lr.w           | s{b|h|w|d}.rl | s{b|h|w|d}  |
|    |            | sc.w.rl       |             | sc.w.rl        |               |             |
|----------------------------------------------------------------------------------------------|
|    |    arch_xchg_acquire       |     arch_cmpxchg_acquire     |    __smp_load_acquire       |
|    |-----------------------------------------------------------------------------------------|
|    | zabha      | !zabha        | zabha+zacas | !(zabha+zacas) | zalasr        | !zalasr     |
| aq |-----------------------------------------------------------------------------------------|
|    |            | lr.w.aq       |             | lr.w.aq        |               | l{b|h|w|d}  |
|    | amoswap.aq | sc.w          | amocas.aq   | sc.w           | l{b|h|w|d}.aq | fence r, rw |
|    |            | (fence r, rw) |             | (fence r, rw)  |               |             |
|----------------------------------------------------------------------------------------------|

(fence rw, w), (fence r, rw) here means such instructions will only be inserted when zalasr is not implemented.