linux-kernel - Re: [PATCH 3/6] KVM: x86/mmu: avoid struct page in MMU

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Message-ID: <0b7f9c30-eb12-35c5-191f-0e8e469e1b88@redhat.com>
Date:   Thu, 24 Jun 2021 12:17:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>, Nicholas Piggin <npiggin@...il.com>
Cc:     Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Huacai Chen <chenhuacai@...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>,
        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 3/6] KVM: x86/mmu: avoid struct page in MMU

On 24/06/21 12:06, Marc Zyngier wrote:
> On Thu, 24 Jun 2021 09:58:00 +0100,
> Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> Excerpts from David Stevens's message of June 24, 2021 1:57 pm:
>>> From: David Stevens <stevensd@...omium.org>
>>>   out_unlock:
>>>   	if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
>>>   		read_unlock(&vcpu->kvm->mmu_lock);
>>>   	else
>>>   		write_unlock(&vcpu->kvm->mmu_lock);
>>> -	kvm_release_pfn_clean(pfn);
>>> +	if (pfnpg.page)
>>> +		put_page(pfnpg.page);
>>>   	return r;
>>>   }
>>
>> How about
>>
>>    kvm_release_pfn_page_clean(pfnpg);
> 
> I'm not sure. I always found kvm_release_pfn_clean() ugly, because it
> doesn't mark the page 'clean'. I find put_page() more correct.
> 
> Something like 'kvm_put_pfn_page()' would make more sense, but I'm so
> bad at naming things that I could just as well call it 'bob()'.

The best way to go would be to get rid of kvm_release_pfn_clean() and 
always go through a pfn_page.  Then we could or could not introduce 
wrappers kvm_put_pfn_page{,_dirty}.

I think for now it's best to limit the churn since these patches will go 
in the stable releases too, and clean up the resulting API once we have 
a clear idea of how all architectures are using kvm_pfn_page.

Paolo