Re: [PATCH v7 09/11] KVM: MMU: introduce kvm_mmu_prepare_zap_obsolete_page

[Gleb Natapov <gleb@redhat.com>]

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@linux.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);
Why do you need hlist_del_init() here? Why not move it into
kvm_mmu_prepare_zap_page() like we discussed it here:
https://patchwork.kernel.org/patch/2580351/ instead of doing
it differently for obsolete and non obsolete pages?

>  	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);
> +
>  	kvm_mmu_page_unlink_children(kvm, sp);
>  	kvm_mmu_unlink_parents(kvm, sp);
>  
> @@ -2099,6 +2104,37 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>  	return ret;
>  }
>  
> +/*
> + * 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.
> + *
> + * Note: if we use this function in for_each_gfn_xxx macros, we should
> + * re-walk the list when it successfully zaps one page.
> + */
> +static int
> +kvm_mmu_prepare_zap_obsolete_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> +				  struct list_head *invalid_list)
> +{
> +	int ret;
> +
> +	WARN_ON(!is_obsolete_sp(kvm, sp));
> +
> +	ret = __kvm_mmu_prepare_zap_page(kvm, sp, false, invalid_list);
> +	if (ret)
> +		hlist_del_init(&sp->hash_link);
Why hlist_del() is not enough?

> +
> +	WARN_ON(ret > 1);
> +	return ret;
> +}
> +
> +static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> +				    struct list_head *invalid_list)
> +{
> +	return __kvm_mmu_prepare_zap_page(kvm, sp, true, invalid_list);
> +}
> +
>  static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>  				    struct list_head *invalid_list)
>  {
> -- 
> 1.7.7.6

--
			Gleb.