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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu, 11 Apr 2024 18:55:44 +0200
Message-ID: <CABgObfYwwXy9gQap-PJyOrVCcUr-VfK90AKNaRe0VO-G00G8SQ@mail.gmail.com>
Subject: Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, 
	Marc Zyngier <maz@kernel.org>, Oliver Upton <oliver.upton@linux.dev>, 
	Tianrui Zhao <zhaotianrui@loongson.cn>, Bibo Mao <maobibo@loongson.cn>, 
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>, Nicholas Piggin <npiggin@gmail.com>, 
	Anup Patel <anup@brainfault.org>, Atish Patra <atishp@atishpatra.org>, 
	Sean Christopherson <seanjc@google.com>, Andrew Morton <akpm@linux-foundation.org>, 
	David Hildenbrand <david@redhat.com>, linux-arm-kernel@lists.infradead.org, 
	kvmarm@lists.linux.dev, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, 
	linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, 
	linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, 
	linux-perf-users@vger.kernel.org, Andrea Arcangeli <aarcange@redhat.com>
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On Mon, Apr 8, 2024 at 3:56=E2=80=AFPM Peter Xu <peterx@redhat.com> wrote:
> Paolo,
>
> I may miss a bunch of details here (as I still remember some change_pte
> patches previously on the list..), however not sure whether we considered
> enable it?  Asked because I remember Andrea used to have a custom tree
> maintaining that part:
>
> https://github.com/aagit/aa/commit/c761078df7a77d13ddfaeebe56a0f4bc128b19=
68

The patch enables it only for KSM, so it would still require a bunch
of cleanups, for example I also would still use set_pte_at() in all
the places that are not KSM. This would at least fix the issue with
the poor documentation of where to use set_pte_at_notify() vs
set_pte_at().

With regard to the implementation, I like the idea of disabling the
invalidation on the MMU notifier side, but I would rather have
MMU_NOTIFIER_CHANGE_PTE as a separate field in the range instead of
overloading the event field.

> Maybe it can't be enabled for some reason that I overlooked in the curren=
t
> tree, or we just decided to not to?

I have just learnt about the patch, nobody had ever mentioned it even
though it's almost 2 years old... It's a lot of code though and no one
has ever reported an issue for over 10 years, so I think it's easiest
to just rip the code out.

Paolo

> Thanks,
>
> --
> Peter Xu
>