Re: [PATCH v3 4/4] membarrier: riscv: Provide core serializing command

[Mathieu Desnoyers]

On 2024-01-24 13:44, Andrea Parri wrote:
> > > +# riscv uses xRET as return from interrupt and to return to user-space.
> > > +#
> > > +# Given that xRET is not core serializing, we rely on FENCE.I for providing
> > > +# core serialization:
> > > +#
> > > +#  - by calling sync_core_before_usermode() on return from interrupt (cf.
> > > +#    ipi_sync_core()),
> > > +#
> > > +#  - via switch_mm() and sync_core_before_usermode() (respectively, for
> > > +#    uthread->uthread and kthread->uthread transitions) to go back to
> > > +#    user-space.
> >
> > I don't quite get the meaning of the sentence above. There seems to be a
> > missing marker before "to go back".
>
> Let's see.  Without the round brackets, the last part becomes:
>
>    - via switch_mm() and sync_core_before_usermode() to go back to
>      user-space.
>
> This is indeed what I meant to say.  What am I missing?

Would it still fit your intent if we say "before returning to
user-space" rather than "to go back to user-space" ?

Because the switch_mm(), for instance, does not happen exactly on
return to user-space, but rather when the scheduler switches tasks.
Therefore, I think that stating that core serialization needs to
happen before returning to user-space is clearer than stating that
it happens "when" we go back to user-space.

Also, on another topic, did you find a way forward with respect of
the different choice of words between the membarrier man page and
documentation vs the RISC-V official semantic with respect to "core
serializing" vs FENCE.I ?

Thanks,

Mathieu


>    Andrea

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com