linux-kernel - Re: [PATCH 1/4] KVM: delete .change

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Date: Thu, 11 Apr 2024 18:55:44 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Nicholas Piggin <npiggin@...il.com>, 
	Anup Patel <anup@...infault.org>, Atish Patra <atishp@...shpatra.org>, 
	Sean Christopherson <seanjc@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, loongarch@...ts.linux.dev, linux-mips@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org, 
	linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback

On Mon, Apr 8, 2024 at 3:56 PM Peter Xu <peterx@...hat.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/c761078df7a77d13ddfaeebe56a0f4bc128b1968

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 current
> 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
>