Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

[Robbin Ehn]

> But fence.i would only have effects on the hart it is being called from, right ?
> What is the use-case for allowing user-space to issue this instruction ?

Right now openjdk uses flush_icache for every cmodx write. And it does
a lot of cmodx.
If the hardware does not require an IPI/icache flushing we still need
to serialize the reader.
(locally stopping out-of-order execution/speculation at least)
And in some cases the reader knows the code stream has been changed
and can emit fence.i itself instead.
So we want the option to emit fence.i directly into the code stream.

As fence.i is an unpriv instruction there is no issue with emitting it.
But we need the assurance that context switching to a new hart does
not eliminate the effects of the fence.i.

/Robbin

>
> One more thing: membarrier(2) sync_core only issues things like "fence.i" on
> the various cores in the system running threads belonging to the process, but
> does not intend to take care of doing any kind of cache invalidation per se
> (e.g. invalidating an address range worth of cache). On ARM, this is done by a
> separate system call (e.g. cacheflush(2)), or can be done by instructions
> available from userspace in some cases.
>
> Do you expect to have a need for flushing only specific icache lines, or is
> the intent to always flush the entire icache ?
>
> >
> > Charlie is volunteering to do the work here, so hopefully we'll have
> > something moving forward.
>
> That's great! I hope my feedback will help.
>
> Thanks,
>
> Mathieu
>
> >
> >>
> >>   Andrea
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>