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

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Message-Id: <1624528342.s2ezcyp90x.astroid@bobo.none>
Date:   Thu, 24 Jun 2021 19:57:57 +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 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.

Thanks,
Nick