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Date:	Sun, 26 May 2013 11:26:49 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v7 03/11] KVM: MMU: fast invalidate all pages

On Fri, May 24, 2013 at 05:23:07PM -0300, Marcelo Tosatti wrote:
> Hi Xiao,
> 
> On Thu, May 23, 2013 at 03:55:52AM +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 obsolete pages (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen)
> > are zapped by using lock-break technique
> > 
> > Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |    2 +
> >  arch/x86/kvm/mmu.c              |   84 +++++++++++++++++++++++++++++++++++++++
> >  arch/x86/kvm/mmu.h              |    1 +
> >  3 files changed, 87 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 f8ca2f3..f302540 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -1838,6 +1838,11 @@ 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,
> > @@ -1900,6 +1905,7 @@ 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;
> > @@ -2070,8 +2076,10 @@ 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);
> >  	if (!sp->root_count) {
> > @@ -4195,6 +4203,82 @@ 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;
> 
> Can you add a comment to list_add(x, active_mmu_pages) callsites
> mentioning this case?
> 
> Because it'll break silently if people do list_add_tail().
> 
> > +		/*
> > +		 * 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;
> > +		 *
> > +		 * Since we are reversely walking the list and the invalid
> > +		 * list will be moved to the head, skip the invalid page
> > +		 * can help us to avoid the infinity list walking.
> > +		 */
> > +		if (sp->role.invalid)
> > +			continue;
> 
> But this allows completing (that is returning), with page that should 
> be zapped still present (even though its invalid).
> 
> Is another pass needed at the end to take care of the invalid pages?
> Which at that point must have their root_count decreased.
> 
It is not different from how it work now. Invalid page can still be not
zapped after zap_all() completes. They are ignored by all relevant code
paths.

> > +
> > +		if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
> > +			kvm_mmu_commit_zap_page(kvm, &invalid_list);
> > +			cond_resched_lock(&kvm->mmu_lock);
> > +			goto restart;
> > +		}
> > +
> > +		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
> > +			goto restart;
> > +	}
> > +
> > +	kvm_mmu_commit_zap_page(kvm, &invalid_list);
> > +}
> > +
> > +/*
> > + * Fast invalidate all shadow pages and use lock-break technique
> > + * to zap obsolete pages.
> > + *
> > + * It's 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.
> > + */
> > +void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm)
> > +{
> > +	spin_lock(&kvm->mmu_lock);
> > +	kvm->arch.mmu_valid_gen++;
> > +
> > +	kvm_zap_obsolete_pages(kvm);
> > +	spin_unlock(&kvm->mmu_lock);
> > +}
> > +
> 
> Also this function should be serialized, that is, should not allow
> simultaneous kvm_mmu_invalidate_zap_all_pages. If thats so
> assert(mutex_is_locked(kvm->lock)) would help.
> 
> Probably fine to have simultaneous users, but not necessary AFAICS.
I raced this point on previous patch submission. The function can be
executed simultaneously only during vmexit when zap_all is called by mm
notifiers. During regular work the function is only called by slot
manipulation functions and those are serialized by slot lock.

--
			Gleb.
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