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

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Message-ID: <fc2a88ed-6a98-857d-bb1f-73260b01ac30@redhat.com>
Date:   Thu, 24 Jun 2021 12:21:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Paul Mackerras <paulus@...abs.org>,
        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

On 24/06/21 12:17, Nicholas Piggin wrote:
>> 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.

Sure, but that would leave us with longer function names 
(gfn_to_pfn_page* instead of gfn_to_pfn*).  So the "safe" use is the one 
that looks worse and the unsafe use is the one that looks safe.

> And are gfn_to_page cases also
> vulernable to the same issue?

No, they're just broken for the VM_IO|VM_PFNMAP case.

Paolo

> 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.