Re: [PATCH v3 12/15] KVM: MMU: fast invalid all shadow pages

[Marcelo Tosatti]

On Tue, Apr 16, 2013 at 02:32:50PM +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 invalid 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.
> 
> The invalid-gen pages (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen)
> are keeped in mmu-cache until page allocator reclaims page.
> 
> If the invalidation is due to memslot changed, its rmap amd lpage-info
> will be freed soon, in order to avoiding use invalid memory, we unmap
> all sptes on its rmap and always reset the large-info all memslots so
> that rmap and lpage info can be safely freed.
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |    2 +
>  arch/x86/kvm/mmu.c              |   85 +++++++++++++++++++++++++++++++++++++-
>  arch/x86/kvm/mmu.h              |    4 ++
>  arch/x86/kvm/x86.c              |    6 +++
>  4 files changed, 94 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1ad9a34..6f8ee18 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -223,6 +223,7 @@ 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
> @@ -531,6 +532,7 @@ 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 9ac584f..12129b7 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1732,6 +1732,11 @@ static struct rmap_operations invalid_rmap_ops = {
>  	.rmap_unmap = kvm_unmap_invalid_rmapp
>  };
>  
> +static void init_invalid_memslot_rmap_ops(struct kvm_memory_slot *slot)
> +{
> +	slot->arch.ops = &invalid_rmap_ops;
> +}
> +
>  typedef void (*handle_rmap_fun)(unsigned long *rmapp, void *data);
>  static void walk_memslot_rmap_nolock(struct kvm_memory_slot *slot,
>  				     handle_rmap_fun fun, void *data)
> @@ -1812,6 +1817,65 @@ void free_meslot_rmap_desc_nolock(struct kvm_memory_slot *slot)
>  	walk_memslot_rmap_nolock(slot, free_rmap_desc_nolock, NULL);
>  }
>  
> +/*
> + * Fast invalid all shadow pages belong to @slot.
> + *
> + * @slot != NULL means the invalidation is caused the memslot specified
> + * by @slot is being deleted, in this case, we should ensure that rmap
> + * and lpage-info of the @slot can not be used after calling the function.
> + * Specially, if @slot is INVALID_ALL_SLOTS, all slots will be deleted
> + * soon, it always happens when kvm is being destroyed.
> + *
> + * @slot == NULL means the invalidation due to other reasons, we need
> + * not care rmap and lpage-info since they are still valid after calling
> + * the function.
> + */
> +void kvm_mmu_invalid_memslot_pages(struct kvm *kvm,
> +				   struct kvm_memory_slot *slot)
> +{
> +	struct kvm_memory_slot *each_slot;
> +
> +	spin_lock(&kvm->mmu_lock);
> +	kvm->arch.mmu_valid_gen++;
> +
> +	if (slot == INVALID_ALL_SLOTS)
> +		kvm_for_each_memslot(each_slot, kvm_memslots(kvm))
> +			init_invalid_memslot_rmap_ops(each_slot);
> +	else if (slot)
> +		init_invalid_memslot_rmap_ops(slot);
> +
> +	/*
> +	 * All shadow paes are invalid, reset the large page info,
> +	 * then we can safely desotry the memslot, it is also good
> +	 * for large page used.
> +	 */
> +	kvm_clear_all_lpage_info(kvm);
> +
> +	/*
> +	 * 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.
> +	 *
> +	 * 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);
> +	spin_unlock(&kvm->mmu_lock);
> +
> +	if (slot == INVALID_ALL_SLOTS)
> +		kvm_for_each_memslot(each_slot, kvm_memslots(kvm))
> +			unmap_memslot_rmap_nolock(kvm, each_slot);
> +	else if (slot)
> +		unmap_memslot_rmap_nolock(kvm, slot);

What is the justification for this? 

> +
> +	/*
> +	 * To ensure that all vcpus and mmu-notify are not clearing
> +	 * spte and rmap entry.
> +	 */
> +	synchronize_srcu_expedited(&kvm->srcu);
> +}
> +
>  #ifdef MMU_DEBUG
>  static int is_empty_shadow_page(u64 *spt)
>  {
> @@ -2219,6 +2283,11 @@ static void clear_sp_write_flooding_count(u64 *spte)
>  	__clear_sp_write_flooding_count(sp);
>  }
>  
> +static bool is_valid_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> +{
> +	return likely(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,
> @@ -2245,6 +2314,9 @@ 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_valid_sp(vcpu->kvm, sp))
> +			continue;
> +
>  		if (!need_sync && sp->unsync)
>  			need_sync = true;
>  
> @@ -2281,6 +2353,7 @@ 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;
> @@ -2451,8 +2524,12 @@ 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);

The rmaps[] arrays linked to !is_valid_sp() shadow pages should not be
accessed (as they have been freed already).

I suppose the is_valid_sp() conditional below should be moved earlier,
before kvm_mmu_unlink_parents or any other rmap access.

This is fine: the !is_valid_sp() shadow pages are only reachable
by SLAB and the hypervisor itself.

>  	kvm_mmu_unlink_parents(kvm, sp);
> -	if (!sp->role.invalid && !sp->role.direct)
> +
> +	if (!sp->role.invalid && !sp->role.direct &&
> +	      /* Invalid-gen pages are not accounted. */
> +	      is_valid_sp(kvm, sp))
>  		unaccount_shadowed(kvm, sp->gfn);
> +
>  	if (sp->unsync)
>  		kvm_unlink_unsync_page(kvm, sp);
>  	if (!sp->root_count) {
> @@ -3256,7 +3333,8 @@ static void mmu_free_roots(struct kvm_vcpu *vcpu)
>  
>  		sp = page_header(root);
>  		--sp->root_count;
> -		if (!sp->root_count && sp->role.invalid) {
> +		if (!is_valid_sp(vcpu->kvm, sp) ||
> +		      (!sp->root_count && sp->role.invalid)) {
>  			kvm_mmu_prepare_zap_page(vcpu->kvm, sp, &invalid_list);
>  			kvm_mmu_commit_zap_page(vcpu->kvm, &invalid_list);

Why can't kvm_mmu_prepare_zap_page+kvm_mmu_commit_zap_page handle
invalid shadow pages?

Perhaps another name is useful to avoid confusion with role.invalid.
Maybe 'defunct'.

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