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Date: Mon, 16 Apr 2012 11:26:19 +0800
From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
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To: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>
CC: Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH v2 03/16] KVM: MMU: properly assert spte on rmap walking
 path
References: <4F87FA69.5060106@linux.vnet.ibm.com> <4F87FBA5.3060203@linux.vnet.ibm.com> <20120414111544.b3a4b9273fe84ddc33cb7e1c@gmail.com>
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On 04/14/2012 10:15 AM, Takuya Yoshikawa wrote:

> On Fri, 13 Apr 2012 18:10:45 +0800
> Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> wrote:
> 
>>  static u64 *rmap_get_next(struct rmap_iterator *iter)
>>  {
>> +	u64 *sptep = NULL;
>> +
>>  	if (iter->desc) {
>>  		if (iter->pos < PTE_LIST_EXT - 1) {
>> -			u64 *sptep;
>> -
>>  			++iter->pos;
>>  			sptep = iter->desc->sptes[iter->pos];
>>  			if (sptep)
>> -				return sptep;
>> +				goto exit;
>>  		}
>>
>>  		iter->desc = iter->desc->more;
>> @@ -1028,11 +1036,14 @@ static u64 *rmap_get_next(struct rmap_iterator *iter)
>>  		if (iter->desc) {
>>  			iter->pos = 0;
>>  			/* desc->sptes[0] cannot be NULL */
>> -			return iter->desc->sptes[iter->pos];
>> +			sptep = iter->desc->sptes[iter->pos];
>> +			goto exit;
>>  		}
>>  	}
>>
>> -	return NULL;
>> +exit:
>> +	WARN_ON(sptep && !is_shadow_present_pte(*sptep));
>> +	return sptep;
>>  }
> 
> This will, probably, again force rmap_get_next function-call even with EPT/NPT:
> CPU cannot skip it by branch prediction.
> 

No, EPT/NPT also needs it.