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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 28 Jul 2020 12:02:08 -0700
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Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for
 retried page fault
To: Nicholas Piggin <npiggin@gmail.com>
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Cc: linux-arch <linux-arch@vger.kernel.org>, Hillf Danton <hdanton@sina.com>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Tue, Jul 28, 2020 at 3:53 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> The quirk is a problem with coprocessor where it's supposed to
> invalidate the translation after a fault but it doesn't, so we can get a
> read-only TLB stuck after something else does a RO->RW upgrade on the
> TLB. Something like that IIRC.  Coprocessors have their own MMU which
> lives in the nest not the core, so you need a global TLB flush to
> invalidate that thing.

So I assumed, but it does seem confused.

Why? Because if there are stale translations on the co-processor,
there's no guarantee that one of the CPU's will have them and take a
fault.

So I'm not seeing why a core CPU doing spurious TLB invalidation would
follow from "stale TLB in the Nest".

If anything, I think "we have a coprocessor that needs to never have
stale TLB entries" would impact the _regular_ TLB invalidates (by
update_mmu_cache()) and perhaps make those more aggressive, exactly
because the coprocessor may not handle the fault as gracefully.

I dunno. I don't know the coprocessor side well enough to judge, I'm
just looking at it from a conceptual standpoint.

          Linus