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

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Date:	Sat, 14 Apr 2012 11:15:44 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 03/16] KVM: MMU: properly assert spte on rmap walking
 path

On Fri, 13 Apr 2012 18:10:45 +0800
Xiao Guangrong <xiaoguangrong@...ux.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.

	Takuya
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