Re: [PATCH v2 03/16] KVM: MMU: properly assert spte on rmap walking path

[Xiao Guangrong]

On 04/14/2012 10:15 AM, Takuya Yoshikawa wrote:

> On Fri, 13 Apr 2012 18:10:45 +0800
> Xiao Guangrong  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.

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Re: [PATCH v2 03/16] KVM: MMU: properly assert spte on rmap walking path

[Takuya Yoshikawa]

On Fri, 13 Apr 2012 18:10:45 +0800
Xiao Guangrong  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.

Takuya
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