Re: [PATCH v2 2/3] KVM: arm64: Add fast path to handle permission relaxation during dirty logging

[Fuad Tabba <tabba@google.com>]

Hi Jing,

On Tue, Jan 18, 2022 at 1:57 AM Jing Zhang <jingzhangos@google.com> wrote:
>
> To reduce MMU lock contention during dirty logging, all permission
> relaxation operations would be performed under read lock.
>
> Signed-off-by: Jing Zhang <jingzhangos@google.com>
> ---
>  arch/arm64/kvm/mmu.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index cafd5813c949..10df5d855d54 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1080,6 +1080,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>         gfn_t gfn;
>         kvm_pfn_t pfn;
>         bool logging_active = memslot_is_logging(memslot);
> +       bool logging_perm_fault = false;
>         unsigned long fault_level = kvm_vcpu_trap_get_fault_level(vcpu);
>         unsigned long vma_pagesize, fault_granule;
>         enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> @@ -1114,6 +1115,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>         if (logging_active) {
>                 force_pte = true;
>                 vma_shift = PAGE_SHIFT;
> +               logging_perm_fault = (fault_status == FSC_PERM && write_fault);
>         } else {
>                 vma_shift = get_vma_page_shift(vma, hva);
>         }
> @@ -1212,7 +1214,15 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>         if (exec_fault && device)
>                 return -ENOEXEC;
>
> -       write_lock(&kvm->mmu_lock);
> +       /*
> +        * To reduce MMU contentions and enhance concurrency during dirty
> +        * logging dirty logging, only acquire read lock for permission
> +        * relaxation.
> +        */

A couple of nits:
"dirty logging" is repeated twice
s/contentions/contention

Other than that,

Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>

Thanks,
/fuad






> +       if (logging_perm_fault)
> +               read_lock(&kvm->mmu_lock);
> +       else
> +               write_lock(&kvm->mmu_lock);
>         pgt = vcpu->arch.hw_mmu->pgt;
>         if (mmu_notifier_retry(kvm, mmu_seq))
>                 goto out_unlock;
> @@ -1271,7 +1281,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>         }
>
>  out_unlock:
> -       write_unlock(&kvm->mmu_lock);
> +       if (logging_perm_fault)
> +               read_unlock(&kvm->mmu_lock);
> +       else
> +               write_unlock(&kvm->mmu_lock);
>         kvm_set_pfn_accessed(pfn);
>         kvm_release_pfn_clean(pfn);
>         return ret != -EAGAIN ? ret : 0;
> --
> 2.34.1.703.g22d0c6ccf7-goog
>
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