Re: [PATCH v7 03/11] KVM: MMU: fast invalidate all pages

[Marcelo Tosatti]

On Sun, May 26, 2013 at 11:26:49AM +0300, Gleb Natapov wrote:
> 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@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >  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.

It is different. kvm_mmu_zap_all() returns with 0 pages at
mmu_active_pages today.

It is probably worthwhile to maintain behaviour (instead of every other
code path does the right thing).

> > 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.

How about fix hypercall vs kvm_set_memory?


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