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SubjectRe: [PATCH 1/7] KVM: MMU: optimize pte write path if don't have protected sp
On 05/15/2011 04:20 PM, Avi Kivity wrote:
> On 05/15/2011 12:35 AM, Xiao Guangrong wrote:
>> Simply return from kvm_mmu_pte_write path if no shadow page is
>> write-protected, then we can avoid to walk all shadow pages and hold
>> mmu-lock
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 2841805..971e2d2 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -498,6 +498,7 @@ static void account_shadowed(struct kvm *kvm, gfn_t gfn)
>> linfo = lpage_info_slot(gfn, slot, i);
>> linfo->write_count += 1;
>> }
>> + atomic_inc(&kvm->arch.indirect_shadow_pages);
>> }
>>
>> static void unaccount_shadowed(struct kvm *kvm, gfn_t gfn)
>> @@ -513,6 +514,7 @@ static void unaccount_shadowed(struct kvm *kvm, gfn_t gfn)
>> linfo->write_count -= 1;
>> WARN_ON(linfo->write_count< 0);
>> }
>> + atomic_dec(&kvm->arch.indirect_shadow_pages);
>> }
>
> These atomic ops are always called from within the spinlock, so we don't need an atomic_t here.
>
> Sorry, I should have noticed this on the first version.

We read indirect_shadow_pages atomically on pte write path, that is allowed out of mmu_lock


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