Re: [PATCH v6 3/7] KVM: MMU: fast invalidate all pages
From: Marcelo Tosatti
Date: Mon May 20 2013 - 16:42:11 EST
On Mon, May 20, 2013 at 11:15:45PM +0300, Gleb Natapov wrote:
> 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@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 same as not using page that is invalid, to not reuse stale
> information. The page may contain ptes that point to invalid slot.
Any pages with stale information will be zapped by kvm_mmu_zap_all().
When that happens, page faults will take place which will automatically
use the new generation number.
So still not clear why is this necessary.
> > 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.
I mean leaving pages which have been "partially deleted" while releasing
mmu_lock. Say for example upon the need to delete a given page you do:
spin_lock(mmu_lock)
shadow page = lookup page on hash list
if (shadow page is not found)
proceed assuming the page has been properly deleted (*)
spin_unlock(mmu_lock)
At (*) we assume it has been 1) deleted and 2) TLB flushed.
So its better to just
if (need_resched()) {
kvm_mmu_complete_zap_page(&list);
cond_resched_lock(&kvm->mmu_lock);
}
If you want to collapse TLB flushes, please do it in a later patch.
> > > + 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.
"
+ /*
+ * 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.
"
What was suggested was... go to phrase which starts with "The only purpose
of the generation number should be to".
The comment quoted here does not match that description.
> > > + *
> > > + * 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.
OK.
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