linux-kernel - Re: [PATCH 1/7] KVM: MMU: optimize pte write path if don't have protected sp

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Message-ID: <4DCF8FC6.8050600@cn.fujitsu.com>
Date:	Sun, 15 May 2011 16:33:10 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [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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