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

[Gleb Natapov]

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.

--
			Gleb.
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