linux-kernel - Re: [PATCH 11/21] KVM: x86/mmu: Zero allocate shadow pages (outside of mmu

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Message-ID: <CANgfPd9vBbX66RYWhW+Lpsrya8Q4SduDHzpbAhAqRyU3i-gHxA@mail.gmail.com>
Date:   Wed, 10 Jun 2020 11:49:02 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Marc Zyngier <maz@...nel.org>, Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Feiner <pfeiner@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Christoffer Dall <christoffer.dall@....com>
Subject: Re: [PATCH 11/21] KVM: x86/mmu: Zero allocate shadow pages (outside
 of mmu_lock)

On Fri, Jun 5, 2020 at 2:39 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Set __GFP_ZERO for the shadow page memory cache and drop the explicit
> clear_page() from kvm_mmu_get_page().  This moves the cost of zeroing a
> page to the allocation time of the physical page, i.e. when topping up
> the memory caches, and thus avoids having to zero out an entire page
> while holding mmu_lock.
>
> Cc: Peter Feiner <pfeiner@...gle.com>
> Cc: Peter Shier <pshier@...gle.com>
> Cc: Junaid Shahid <junaids@...gle.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Suggested-by: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Reviewed-by: Ben Gardon <bgardon@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6b0ec9060786..a8f8eebf67df 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2545,7 +2545,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>                 if (level > PG_LEVEL_4K && need_sync)
>                         flush |= kvm_sync_pages(vcpu, gfn, &invalid_list);
>         }
> -       clear_page(sp->spt);
>         trace_kvm_mmu_get_page(sp, true);
>
>         kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush);
> @@ -5687,6 +5686,8 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
>         vcpu->arch.mmu_page_header_cache.kmem_cache = mmu_page_header_cache;
>         vcpu->arch.mmu_page_header_cache.gfp_zero = __GFP_ZERO;
>
> +       vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> +
>         vcpu->arch.mmu = &vcpu->arch.root_mmu;
>         vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;
>
> --
> 2.26.0
>