linux-kernel - Re: [PATCH 07/21] KVM: x86/mmu: Topup memory caches after walking GVA->GPA

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Date:   Wed, 10 Jun 2020 15:34:28 -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 07/21] KVM: x86/mmu: Topup memory caches after walking GVA->GPA

On Fri, Jun 5, 2020 at 2:39 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Topup memory caches after walking the GVA->GPA translation during a
> shadow page fault, there is no need to ensure the caches are full when
> walking the GVA.  As of commit f5a1e9f89504f ("KVM: MMU: remove call
> to kvm_mmu_pte_write from walk_addr"), the FNAME(walk_addr) flow no
> longer add rmaps via kvm_mmu_pte_write().
>
> This avoids allocating memory in the case that the GVA is unmapped in
> the guest, and also provides a paper trail of why/when the memory caches
> need to be filled.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Reviewed-by: Ben Gardon <bgardon@...gle.com>
> ---
>  arch/x86/kvm/mmu/paging_tmpl.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 38c576495048..3de32122f601 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -791,10 +791,6 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t addr, u32 error_code,
>
>         pgprintk("%s: addr %lx err %x\n", __func__, addr, error_code);
>
> -       r = mmu_topup_memory_caches(vcpu);
> -       if (r)
> -               return r;
> -
>         /*
>          * If PFEC.RSVD is set, this is a shadow page fault.
>          * The bit needs to be cleared before walking guest page tables.
> @@ -822,6 +818,10 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t addr, u32 error_code,
>                 return RET_PF_EMULATE;
>         }
>
> +       r = mmu_topup_memory_caches(vcpu);
> +       if (r)
> +               return r;
> +
>         vcpu->arch.write_fault_to_shadow_pgtable = false;
>
>         is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,
> --
> 2.26.0
>