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Date:   Thu, 24 Jun 2021 20:17:42 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to_pfn
To:     Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Paul Mackerras <paulus@ozlabs.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Stevens <stevensd@chromium.org>,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>
Cc:     Alexandru Elisei <alexandru.elisei@arm.com>,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        intel-gvt-dev@lists.freedesktop.org,
        James Morse <james.morse@arm.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvmarm@lists.cs.columbia.edu,
        kvm-ppc@vger.kernel.org, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Sean Christopherson <seanjc@google.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Will Deacon <will@kernel.org>
References: <20210624035749.4054934-1-stevensd@google.com>
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        <1624524331.zsin3qejl9.astroid@bobo.none>
        <201b68a7-10ea-d656-0c1e-5511b1f22674@redhat.com>
        <1624528342.s2ezcyp90x.astroid@bobo.none>
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Excerpts from Nicholas Piggin's message of June 24, 2021 7:57 pm:
> Excerpts from Paolo Bonzini's message of June 24, 2021 7:42 pm:
>> On 24/06/21 10:52, Nicholas Piggin wrote:
>>>> For now, wrap all calls to gfn_to_pfn functions in the new helper
>>>> function. Callers which don't need the page struct will be updated in
>>>> follow-up patches.
>>> Hmm. You mean callers that do need the page will be updated? Normally
>>> if there will be leftover users that don't need the struct page then
>>> you would go the other way and keep the old call the same, and add a ne=
w
>>> one (gfn_to_pfn_page) just for those that need it.
>>=20
>> Needing kvm_pfn_page_unwrap is a sign that something might be buggy, so=20
>> it's a good idea to move the short name to the common case and the ugly=20
>> kvm_pfn_page_unwrap(gfn_to_pfn(...)) for the weird one.  In fact I'm not=
=20
>> sure there should be any kvm_pfn_page_unwrap in the end.
>=20
> If all callers were updated that is one thing, but from the changelog
> it sounds like that would not happen and there would be some gfn_to_pfn
> users left over.
>=20
> But yes in the end you would either need to make gfn_to_pfn never return
> a page found via follow_pte, or change all callers to the new way. If=20
> the plan is for the latter then I guess that's fine.

Actually in that case anyway I don't see the need -- the existence of
gfn_to_pfn is enough to know it might be buggy. It can just as easily
be grepped for as kvm_pfn_page_unwrap. And are gfn_to_page cases also
vulernable to the same issue?

So I think it could be marked deprecated or something if not everything=20
will be converted in the one series, and don't need to touch all that=20
arch code with this patch.

Thanks,
Nick