Re: [PATCH 2/2] KVM: x86: Optimize mmio spte zapping when creating/moving memslot

[Gleb Natapov <gleb@redhat.com>]

On Tue, Mar 12, 2013 at 05:45:30PM +0900, Takuya Yoshikawa wrote:
> When we create or move a memory slot, we need to zap mmio sptes.
> Currently, zap_all() is used for this and this is causing two problems:
>  - extra page faults after zapping mmu pages
>  - long mmu_lock hold time during zapping mmu pages
> 
> For the latter, Marcelo reported a disastrous mmu_lock hold time during
> hot-plug, which made the guest unresponsive for a long time.
> 
> This patch takes a simple way to fix these problems: do not zap mmu
> pages unless they are marked mmio cached.  On our test box, this took
> only 50us for the 4GB guest and we did not see ms of mmu_lock hold time
> any more.
> 
> Note that we still need to do zap_all() for other cases.  So another
> work is also needed: Xiao's work may be the one.
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
> ---
>  arch/x86/include/asm/kvm_host.h |    1 +
>  arch/x86/kvm/mmu.c              |   18 ++++++++++++++++++
>  arch/x86/kvm/x86.c              |    2 +-
>  3 files changed, 20 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b84310a..028b03f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -768,6 +768,7 @@ void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
>  				     struct kvm_memory_slot *slot,
>  				     gfn_t gfn_offset, unsigned long mask);
>  void kvm_mmu_zap_all(struct kvm *kvm);
> +void kvm_mmu_zap_mmio_sptes(struct kvm *kvm);
>  unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
>  void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
>  
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index de45ec1..c1a9b7b 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4189,6 +4189,24 @@ restart:
>  	spin_unlock(&kvm->mmu_lock);
>  }
>  
> +void kvm_mmu_zap_mmio_sptes(struct kvm *kvm)
> +{
> +	struct kvm_mmu_page *sp, *node;
> +	LIST_HEAD(invalid_list);
> +
> +	spin_lock(&kvm->mmu_lock);
> +restart:
> +	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
> +		if (!sp->mmio_cached)
> +			continue;
> +		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
> +			goto restart;
> +	}
> +
> +	kvm_mmu_commit_zap_page(kvm, &invalid_list);
> +	spin_unlock(&kvm->mmu_lock);
> +}
> +
>  static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
>  {
>  	struct kvm *kvm;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 35b4912..16b6df2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6969,7 +6969,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  	 * mmio sptes.
>  	 */
>  	if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
I wonder why check for KVM_MR_MOVE here. For KVM_MR_MOVE
kvm_mmu_zap_all() should be called and it is indeed called by the common code.

> -		kvm_mmu_zap_all(kvm);
> +		kvm_mmu_zap_mmio_sptes(kvm);
>  		kvm_reload_remote_mmus(kvm);
>  	}
>  }
> -- 
> 1.7.5.4

--
			Gleb.