Received-SPF: pass (google.com: domain of linux-kernel+bounces-141489-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Date: Thu, 11 Apr 2024 14:47:56 -0400
From: Peter Xu <peterx@redhat.com>
To: Paolo Bonzini <pbonzini@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>
Subject: Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback
Message-ID: <ZhgwXGuUeXbFjrCp@x1n>
References: <20240405115815.3226315-1-pbonzini@redhat.com>
 <20240405115815.3226315-2-pbonzini@redhat.com>
 <ZhP3hDhe2Qwo9oCL@x1n>
 <CABgObfYwwXy9gQap-PJyOrVCcUr-VfK90AKNaRe0VO-G00G8SQ@mail.gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CABgObfYwwXy9gQap-PJyOrVCcUr-VfK90AKNaRe0VO-G00G8SQ@mail.gmail.com>

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@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/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