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

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Date: Thu, 11 Apr 2024 14:47:56 -0400
From: Peter Xu <peterx@...hat.com>
To: Paolo Bonzini <pbonzini@...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 Thu, Apr 11, 2024 at 06:55:44PM +0200, Paolo Bonzini wrote:
> 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.

Right, it was pretty old and I have no idea if that was discussed or
published before..  It would be better to have discussed this earlier.

As long as we have a decision with that being aware and in mind, then it
looks fine to me to take either way to go, and I also agree either way is
better than keep the status quo.

I also have Andrea copied anyway when I replied, so I guess he should be
aware of this and he can chim in anytime.

Thanks!

-- 
Peter Xu