Re: [PATCH v6 3/7] KVM: MMU: fast invalidate all pages

[Marcelo Tosatti]

On Fri, May 17, 2013 at 05:12:58AM +0800, Xiao Guangrong wrote:
> The current kvm_mmu_zap_all is really slow - it is holding mmu-lock to
> walk and zap all shadow pages one by one, also it need to zap all guest
> page's rmap and all shadow page's parent spte list. Particularly, things
> become worse if guest uses more memory or vcpus. It is not good for
> scalability
> 
> In this patch, we introduce a faster way to invalidate all shadow pages.
> KVM maintains a global mmu invalid generation-number which is stored in
> kvm->arch.mmu_valid_gen and every shadow page stores the current global
> generation-number into sp->mmu_valid_gen when it is created
> 
> When KVM need zap all shadow pages sptes, it just simply increase the
> global generation-number then reload root shadow pages on all vcpus.
> Vcpu will create a new shadow page table according to current kvm's
> generation-number. It ensures the old pages are not used any more.
> Then the invalid-gen pages (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen)
> are zapped by using lock-break technique
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |    2 +
>  arch/x86/kvm/mmu.c              |  103 +++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/mmu.h              |    1 +
>  3 files changed, 106 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3741c65..bff7d46 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -222,6 +222,7 @@ struct kvm_mmu_page {
>  	int root_count;          /* Currently serving as active root */
>  	unsigned int unsync_children;
>  	unsigned long parent_ptes;	/* Reverse mapping for parent_pte */
> +	unsigned long mmu_valid_gen;
>  	DECLARE_BITMAP(unsync_child_bitmap, 512);
>  
>  #ifdef CONFIG_X86_32
> @@ -529,6 +530,7 @@ struct kvm_arch {
>  	unsigned int n_requested_mmu_pages;
>  	unsigned int n_max_mmu_pages;
>  	unsigned int indirect_shadow_pages;
> +	unsigned long mmu_valid_gen;
>  	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
>  	/*
>  	 * Hash table of struct kvm_mmu_page.
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 682ecb4..891ad2c 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1839,6 +1839,11 @@ static void clear_sp_write_flooding_count(u64 *spte)
>  	__clear_sp_write_flooding_count(sp);
>  }
>  
> +static bool is_obsolete_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> +{
> +	return unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);
> +}
> +
>  static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  					     gfn_t gfn,
>  					     gva_t gaddr,
> @@ -1865,6 +1870,9 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  		role.quadrant = quadrant;
>  	}
>  	for_each_gfn_sp(vcpu->kvm, sp, gfn) {
> +		if (is_obsolete_sp(vcpu->kvm, sp))
> +			continue;
> +

Whats the purpose of not using pages which are considered "obsolete" ?

The only purpose of the generation number should be to guarantee forward
progress of kvm_mmu_zap_all in face of allocators (kvm_mmu_get_page).

That is, on allocation you store the generation number on the shadow page.
The only purpose of that number in the shadow page is so that
kvm_mmu_zap_all can skip deleting shadow pages which have been created
after kvm_mmu_zap_all began operation.

>  		if (!need_sync && sp->unsync)
>  			need_sync = true;
>  
> @@ -1901,6 +1909,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  
>  		account_shadowed(vcpu->kvm, gfn);
>  	}
> +	sp->mmu_valid_gen = vcpu->kvm->arch.mmu_valid_gen;
>  	init_shadow_page_table(sp);
>  	trace_kvm_mmu_get_page(sp, true);
>  	return sp;
> @@ -2071,8 +2080,10 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>  	ret = mmu_zap_unsync_children(kvm, sp, invalid_list);
>  	kvm_mmu_page_unlink_children(kvm, sp);
>  	kvm_mmu_unlink_parents(kvm, sp);
> +
>  	if (!sp->role.invalid && !sp->role.direct)
>  		unaccount_shadowed(kvm, sp->gfn);
> +
>  	if (sp->unsync)
>  		kvm_unlink_unsync_page(kvm, sp);
>  
> @@ -4196,6 +4207,98 @@ restart:
>  	spin_unlock(&kvm->mmu_lock);
>  }
>  
> +static void kvm_zap_obsolete_pages(struct kvm *kvm)
> +{
> +	struct kvm_mmu_page *sp, *node;
> +	LIST_HEAD(invalid_list);
> +
> +restart:
> +	list_for_each_entry_safe_reverse(sp, node,
> +	      &kvm->arch.active_mmu_pages, link) {
> +		/*
> +		 * No obsolete page exists before new created page since
> +		 * active_mmu_pages is the FIFO list.
> +		 */
> +		if (!is_obsolete_sp(kvm, sp))
> +			break;
> +
> +		/*
> +		 * Do not repeatedly zap a root page to avoid unnecessary
> +		 * KVM_REQ_MMU_RELOAD, otherwise we may not be able to
> +		 * progress:
> +		 *    vcpu 0                        vcpu 1
> +		 *                         call vcpu_enter_guest():
> +		 *                            1): handle KVM_REQ_MMU_RELOAD
> +		 *                                and require mmu-lock to
> +		 *                                load mmu
> +		 * repeat:
> +		 *    1): zap root page and
> +		 *        send KVM_REQ_MMU_RELOAD
> +		 *
> +		 *    2): if (cond_resched_lock(mmu-lock))
> +		 *
> +		 *                            2): hold mmu-lock and load mmu
> +		 *
> +		 *                            3): see KVM_REQ_MMU_RELOAD bit
> +		 *                                on vcpu->requests is set
> +		 *                                then return 1 to call
> +		 *                                vcpu_enter_guest() again.
> +		 *            goto repeat;
> +		 *
> +		 */
> +		if (sp->role.invalid)
> +			continue;
> +		/*
> +		 * Need not flush tlb since we only zap the sp with invalid
> +		 * generation number.
> +		 */
> +		if (cond_resched_lock(&kvm->mmu_lock))
> +			goto restart;

Please don't do this, just release all pages and flush the TLB when
dropping mmu_lock instead. This means for other mmu_lock users is that
there is a consistent state.

> +		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
> +			goto restart;
> +	}
> +
> +	/*
> +	 * Should flush tlb before free page tables since lockless-walking
> +	 * may use the pages.
> +	 */
> +	kvm_mmu_commit_zap_page(kvm, &invalid_list);
> +}
> +
> +/*
> + * Fast invalidate all shadow pages.
> + *
> + * @zap_obsolete_pages indicates whether all the obsolete pages should
> + * be zapped. This is required when memslot is being deleted or VM is
> + * being destroyed, in these cases, we should ensure that KVM MMU does
> + * not use any resource of the being-deleted slot or all slots after
> + * calling the function.
> + *
> + * @zap_obsolete_pages == false means the caller just wants to flush all
> + * shadow page tables.
> + */
> +void kvm_mmu_invalidate_all_pages(struct kvm *kvm, bool zap_obsolete_pages)
> +{
> +	spin_lock(&kvm->mmu_lock);
> +	kvm->arch.mmu_valid_gen++;
> +
> +	/*
> +	 * Notify all vcpus to reload its shadow page table
> +	 * and flush TLB. Then all vcpus will switch to new
> +	 * shadow page table with the new mmu_valid_gen.

Only if you zap the roots, which we agreed would be a second step, after
being understood its necessary.

> +	 *
> +	 * Note: we should do this under the protection of
> +	 * mmu-lock, otherwise, vcpu would purge shadow page
> +	 * but miss tlb flush.
> +	 */
> +	kvm_reload_remote_mmus(kvm);
> +
> +	if (zap_obsolete_pages)
> +		kvm_zap_obsolete_pages(kvm);

Please don't condition behaviour on parameters like this, its confusing. 
Can probably just use

spin_lock(&kvm->mmu_lock);
kvm->arch.mmu_valid_gen++;
kvm_reload_remote_mmus(kvm);
spin_unlock(&kvm->mmu_lock);

Directly when needed.

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