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

[Gleb Natapov <gleb@redhat.com>]

On Mon, May 20, 2013 at 04:46:24PM -0300, Marcelo Tosatti wrote:
> 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@linux.vnet.ibm.com>
> > ---
> >  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 same as not using page that is invalid, to not reuse stale
information. The page may contain ptes that point to invalid slot.

> 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.
You mean call kvm_mmu_commit_zap_page() on  current invalid_list before restart?
If yes, I agree.

> 
> > +		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.
> 
I've lost you here. The patch implement what was agreed upon.

> > +	 *
> > +	 * 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.

Please, no. Better have one place where mmu_valid_gen is
handled. If parameter is confusing better have two functions:
kvm_mmu_invalidate_all_pages() and kvm_mmu_invalidate_zap_all_pages(),
but to minimize code duplication they will call the same function with
a parameter anyway, so I am not sure this is a win.

--
			Gleb.