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Date: Wed, 17 Apr 2013 21:05:50 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc: gleb@...hat.com, avi.kivity@...il.com,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 12/15] KVM: MMU: fast invalid all shadow pages
On Tue, Apr 16, 2013 at 02:32:50PM +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 invalid 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.
>
> The invalid-gen pages (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen)
> are keeped in mmu-cache until page allocator reclaims page.
>
> If the invalidation is due to memslot changed, its rmap amd lpage-info
> will be freed soon, in order to avoiding use invalid memory, we unmap
> all sptes on its rmap and always reset the large-info all memslots so
> that rmap and lpage info can be safely freed.
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
> arch/x86/include/asm/kvm_host.h | 2 +
> arch/x86/kvm/mmu.c | 85 +++++++++++++++++++++++++++++++++++++-
> arch/x86/kvm/mmu.h | 4 ++
> arch/x86/kvm/x86.c | 6 +++
> 4 files changed, 94 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 1ad9a34..6f8ee18 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -223,6 +223,7 @@ 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
> @@ -531,6 +532,7 @@ 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 9ac584f..12129b7 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1732,6 +1732,11 @@ static struct rmap_operations invalid_rmap_ops = {
> .rmap_unmap = kvm_unmap_invalid_rmapp
> };
>
> +static void init_invalid_memslot_rmap_ops(struct kvm_memory_slot *slot)
> +{
> + slot->arch.ops = &invalid_rmap_ops;
> +}
> +
> typedef void (*handle_rmap_fun)(unsigned long *rmapp, void *data);
> static void walk_memslot_rmap_nolock(struct kvm_memory_slot *slot,
> handle_rmap_fun fun, void *data)
> @@ -1812,6 +1817,65 @@ void free_meslot_rmap_desc_nolock(struct kvm_memory_slot *slot)
> walk_memslot_rmap_nolock(slot, free_rmap_desc_nolock, NULL);
> }
>
> +/*
> + * Fast invalid all shadow pages belong to @slot.
> + *
> + * @slot != NULL means the invalidation is caused the memslot specified
> + * by @slot is being deleted, in this case, we should ensure that rmap
> + * and lpage-info of the @slot can not be used after calling the function.
> + * Specially, if @slot is INVALID_ALL_SLOTS, all slots will be deleted
> + * soon, it always happens when kvm is being destroyed.
> + *
> + * @slot == NULL means the invalidation due to other reasons, we need
> + * not care rmap and lpage-info since they are still valid after calling
> + * the function.
> + */
> +void kvm_mmu_invalid_memslot_pages(struct kvm *kvm,
> + struct kvm_memory_slot *slot)
> +{
> + struct kvm_memory_slot *each_slot;
> +
> + spin_lock(&kvm->mmu_lock);
> + kvm->arch.mmu_valid_gen++;
> +
> + if (slot == INVALID_ALL_SLOTS)
> + kvm_for_each_memslot(each_slot, kvm_memslots(kvm))
> + init_invalid_memslot_rmap_ops(each_slot);
> + else if (slot)
> + init_invalid_memslot_rmap_ops(slot);
> +
> + /*
> + * All shadow paes are invalid, reset the large page info,
> + * then we can safely desotry the memslot, it is also good
> + * for large page used.
> + */
> + kvm_clear_all_lpage_info(kvm);
> +
> + /*
> + * 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.
> + *
> + * 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);
> + spin_unlock(&kvm->mmu_lock);
> +
> + if (slot == INVALID_ALL_SLOTS)
> + kvm_for_each_memslot(each_slot, kvm_memslots(kvm))
> + unmap_memslot_rmap_nolock(kvm, each_slot);
> + else if (slot)
> + unmap_memslot_rmap_nolock(kvm, slot);
What is the justification for this?
> +
> + /*
> + * To ensure that all vcpus and mmu-notify are not clearing
> + * spte and rmap entry.
> + */
> + synchronize_srcu_expedited(&kvm->srcu);
> +}
> +
> #ifdef MMU_DEBUG
> static int is_empty_shadow_page(u64 *spt)
> {
> @@ -2219,6 +2283,11 @@ static void clear_sp_write_flooding_count(u64 *spte)
> __clear_sp_write_flooding_count(sp);
> }
>
> +static bool is_valid_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> +{
> + return likely(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,
> @@ -2245,6 +2314,9 @@ 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_valid_sp(vcpu->kvm, sp))
> + continue;
> +
> if (!need_sync && sp->unsync)
> need_sync = true;
>
> @@ -2281,6 +2353,7 @@ 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;
> @@ -2451,8 +2524,12 @@ 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);
The rmaps[] arrays linked to !is_valid_sp() shadow pages should not be
accessed (as they have been freed already).
I suppose the is_valid_sp() conditional below should be moved earlier,
before kvm_mmu_unlink_parents or any other rmap access.
This is fine: the !is_valid_sp() shadow pages are only reachable
by SLAB and the hypervisor itself.
> kvm_mmu_unlink_parents(kvm, sp);
> - if (!sp->role.invalid && !sp->role.direct)
> +
> + if (!sp->role.invalid && !sp->role.direct &&
> + /* Invalid-gen pages are not accounted. */
> + is_valid_sp(kvm, sp))
> unaccount_shadowed(kvm, sp->gfn);
> +
> if (sp->unsync)
> kvm_unlink_unsync_page(kvm, sp);
> if (!sp->root_count) {
> @@ -3256,7 +3333,8 @@ static void mmu_free_roots(struct kvm_vcpu *vcpu)
>
> sp = page_header(root);
> --sp->root_count;
> - if (!sp->root_count && sp->role.invalid) {
> + if (!is_valid_sp(vcpu->kvm, sp) ||
> + (!sp->root_count && sp->role.invalid)) {
> kvm_mmu_prepare_zap_page(vcpu->kvm, sp, &invalid_list);
> kvm_mmu_commit_zap_page(vcpu->kvm, &invalid_list);
Why can't kvm_mmu_prepare_zap_page+kvm_mmu_commit_zap_page handle
invalid shadow pages?
Perhaps another name is useful to avoid confusion with role.invalid.
Maybe 'defunct'.
--
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