linux-kernel - Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to

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Date:   Thu, 24 Jun 2021 20:17:42 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Paul Mackerras <paulus@...abs.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Stevens <stevensd@...omium.org>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>
Cc:     Alexandru Elisei <alexandru.elisei@....com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org,
        James Morse <james.morse@....com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvmarm@...ts.cs.columbia.edu,
        kvm-ppc@...r.kernel.org, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Sean Christopherson <seanjc@...gle.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to_pfn

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 new
>>> one (gfn_to_pfn_page) just for those that need it.
>> 
>> Needing kvm_pfn_page_unwrap is a sign that something might be buggy, so 
>> it's a good idea to move the short name to the common case and the ugly 
>> kvm_pfn_page_unwrap(gfn_to_pfn(...)) for the weird one.  In fact I'm not 
>> sure there should be any kvm_pfn_page_unwrap in the end.
> 
> 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.
> 
> 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 
> 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 
will be converted in the one series, and don't need to touch all that 
arch code with this patch.

Thanks,
Nick