linux-kernel - Re: [PATCH v7 09/11] KVM: MMU: introduce kvm_mmu_prepare_zap_obsolete

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Date:	Mon, 27 May 2013 21:13:02 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 09/11] KVM: MMU: introduce
 kvm_mmu_prepare_zap_obsolete_page

On Thu, May 23, 2013 at 03:55:58AM +0800, Xiao Guangrong wrote:
> It is only used to zap the obsolete page. Since the obsolete page
> will not be used, we need not spend time to find its unsync children
> out. Also, we delete the page from shadow page cache so that the page
> is completely isolated after call this function.
> 
> The later patch will use it to collapse tlb flushes
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kvm/mmu.c |   46 +++++++++++++++++++++++++++++++++++++++++-----
>  1 files changed, 41 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 9b57faa..e676356 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1466,7 +1466,7 @@ static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
>  static void kvm_mmu_free_page(struct kvm_mmu_page *sp)
>  {
>  	ASSERT(is_empty_shadow_page(sp->spt));
> -	hlist_del(&sp->hash_link);
> +	hlist_del_init(&sp->hash_link);
>  	list_del(&sp->link);
>  	free_page((unsigned long)sp->spt);
>  	if (!sp->role.direct)
> @@ -2069,14 +2069,19 @@ static int mmu_zap_unsync_children(struct kvm *kvm,
>  	return zapped;
>  }
>  
> -static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> -				    struct list_head *invalid_list)
> +static int
> +__kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> +			   bool zap_unsync_children,
> +			   struct list_head *invalid_list)
>  {
> -	int ret;
> +	int ret = 0;
>  
>  	trace_kvm_mmu_prepare_zap_page(sp);
>  	++kvm->stat.mmu_shadow_zapped;
> -	ret = mmu_zap_unsync_children(kvm, sp, invalid_list);
> +
> +	if (likely(zap_unsync_children))
> +		ret = mmu_zap_unsync_children(kvm, sp, invalid_list);
> +

Why is this an important case to be optimized?

1) shadow is the uncommon, obsolete case.
2) mmu_zap_unsync_children has

        if (parent->role.level == PT_PAGE_TABLE_LEVEL)
                return 0;

So the large majority of pages are already optimized.

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