Re: [PATCH v5 3/8] KVM: MMU: fast invalidate all pages

From: Gleb Natapov
Date: Thu May 16 2013 - 11:57:51 EST

On Thu, May 16, 2013 at 04:43:21PM +0300, Gleb Natapov wrote:
> On Thu, May 16, 2013 at 09:25:28PM +0800, Xiao Guangrong wrote:
> > On 05/16/2013 08:43 PM, Gleb Natapov wrote:
> > > On Thu, May 16, 2013 at 08:17:48PM +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 | 98 +++++++++++++++++++++++++++++++++++++++
> > >> arch/x86/kvm/mmu.h | 2 +
> > >> 3 files changed, 102 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..d9343fe 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;
> > >> +
> > >> 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 +4207,93 @@ restart:
> > >> spin_unlock(&kvm->mmu_lock);
> > >> }
> > >>
> > >> +static void zap_invalid_pages(struct kvm *kvm)
> > >> +{
> > >> + struct kvm_mmu_page *sp, *node;
> > >> + LIST_HEAD(invalid_list);
> > >> +
> > >> +restart:
> > >> + list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
> > >> + if (!is_obsolete_sp(kvm, sp))
> > >> + continue;
> > > What if we save kvm->arch.active_mmu_pages on the stack and init
> > > kvm->arch.active_mmu_pages to be empty at the entrance to
> > > zap_invalid_pages(). This loop will iterate over saved list. This will
> > > allow us to drop the is_obsolete_sp() check and will save time since we
> > > will not be iterating over newly created sps.
> >
> > This idea is really smart.
> >
> > It also seems tricky, vcpu can see the page in its page table and hash table but
> > it has already been deleted from kvm->active_list, but i do not see any issue.
> >
> Paolo pointed that it breaks mmu pages accounting. Can be solved, but
> not trivial.
>
> > Hmm, can we walk kvm->ative_mmu_pages from tail to head then break the walking
> > if we meet the sp->valid_gen == kvm->valid_gen? This way also can skip walking
> > new created sps and more straight.
> >
> Yes, that should be better than walking it from the start each time.
>
One more thought. With current patch if zap_invalid_page() will be
called second time while another zap_invalid_page() is still running
(can that happen?) they will both run concurrently fighting for the
mmu_lock. Is this a problem?

--
Gleb.
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