Re: [PATCH] riscv: Fix text patching when icache flushes use IPIs

From: Andrea Parri
Date: Thu Feb 08 2024 - 09:14:48 EST

> I did not even think of that, and it actually makes sense so I'll go
> with what you propose: I'll replace atomic_inc() with
> atomic_inc_return_release(). And I'll add the following comment if
> that's ok with you:
> "Make sure the patching store is effective *before* we increment the
> counter which releases all waiting cpus"

Yes, this sounds good to me.

> Honestly, I looked at it one minute, did not understand its purpose
> and said to myself "ok that can't hurt anyway, I may be missing
> something".
> FWIW, I see that arm64 uses isb() here. If you don't see its purpose,
> I'll remove it (here and where I copied it).

Removing the smp_mb() (and keeping the local_flush_icache_all()) seems
fine to me; thanks for the confirmation.

> > On a last topic, although somehow orthogonal to the scope of this patch,
> > I'm not sure the patch_{map,unmap}() dance in our patch_insn_write() is
> > correct: I can see why we may want (need to do) the local TLB flush be-
> > fore returning from patch_{map,unmap}(), but does a local flush suffice?
> > For comparison, arm64 seems to go through a complete dsb-tlbi-dsb(-isb)
> > sequence in their unmapping stage (and apparently relying on "no caching
> > of invalid ptes" in their mapping stage). Of course, "broadcasting" our
> > (riscv's) TLB invalidations will necessary introduce some complexity...
> >
> > Thoughts?
> To avoid remote TLBI, could we simply disable the preemption before
> the first patch_map()? arm64 disables the irqs, but that seems
> overkill to me, but maybe I'm missing something again?

Mmh, I'm afraid this will require more thinking/probing on my end (not
really "the expert" of the codebase at stake...). Maybe the ftrace
reviewers will provide further ideas/suggestions for us to brainstorm.