Re: [Qemu-devel] [PATCH 4/4] KVM: MMU: fast cleanup D bit based on fast write protect

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Jan 17, 2019 at 01:55:31PM +0000, Zhuangyanying wrote:
> From: Zhuang Yanying <ann.zhuangyanying@huawei.com>
> 
> When live-migration with large-memory guests, vcpu may hang for a long
> time while starting migration, such as 9s for 2T
> (linux-5.0.0-rc2+qemu-3.1.0).
> The reason is memory_global_dirty_log_start() taking too long, and the
> vcpu is waiting for BQL. The page-by-page D bit clearup is the main time
> consumption. I think that the idea of "KVM: MMU: fast write protect" by
> xiaoguangrong, especially the function kvm_mmu_write_protect_all_pages(),
> is very helpful. After a little modifcation, on his patch, can solve
> this problem, 9s to 0.5s.
> 
> At the beginning of live migration, write protection is only applied to the
> top-level SPTE. Then the write from vm trigger the EPT violation, with
> for_each_shadow_entry write protection is performed at dirct_map.
> Finally the Dirty bit of the target page(at level 1 page table) is
> cleared, and the dirty page tracking is started. Of coure, the page where
> GPA is located is marked dirty when mmu_set_spte.
> A similar implementation on xen, just emt instead of write protection.
> 
> Signed-off-by: Zhuang Yanying <ann.zhuangyanying@huawei.com>
> ---
>  arch/x86/kvm/mmu.c     | 8 +++++---
>  arch/x86/kvm/vmx/vmx.c | 3 +--
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 047b897..a18bcc0 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3257,7 +3257,10 @@ static bool mmu_load_shadow_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  			break;
>  
>  		if (is_last_spte(spte, sp->role.level)) {
> -			flush |= spte_write_protect(sptep, false);
> +			if (sp->role.level == PT_PAGE_TABLE_LEVEL)
> +				flush |= spte_clear_dirty(sptep);
> +			else
> +				flush |= spte_write_protect(sptep, false);
>  			continue;
>  		}
>  
> @@ -6114,7 +6117,6 @@ void kvm_mmu_write_protect_all_pages(struct kvm *kvm, bool write_protect)
>  {
>  	u64 wp_all_indicator, kvm_wp_all_gen;
>  
> -	mutex_lock(&kvm->slots_lock);
>  	wp_all_indicator = get_write_protect_all_indicator(kvm);
>  	kvm_wp_all_gen = get_write_protect_all_gen(wp_all_indicator);
>  
> @@ -6134,8 +6136,8 @@ void kvm_mmu_write_protect_all_pages(struct kvm *kvm, bool write_protect)
>  	 */
>  	if (write_protect)
>  		kvm_reload_remote_mmus(kvm);
> -	mutex_unlock(&kvm->slots_lock);

Why is the lock removed?  And why was it added in the first place?

>  }
> +EXPORT_SYMBOL_GPL(kvm_mmu_write_protect_all_pages);
>  
>  static unsigned long
>  mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f6915f1..5236a07 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7180,8 +7180,7 @@ static void vmx_sched_in(struct kvm_vcpu *vcpu, int cpu)
>  static void vmx_slot_enable_log_dirty(struct kvm *kvm,
>  				     struct kvm_memory_slot *slot)
>  {
> -	kvm_mmu_slot_leaf_clear_dirty(kvm, slot);
> -	kvm_mmu_slot_largepage_remove_write_access(kvm, slot);
> +	kvm_mmu_write_protect_all_pages(kvm, true);

What's the purpose of having @write_protect if
kvm_mmu_write_protect_all_pages() is only ever called to enable
protection?  If there's no known scenario where write protection is
explicitly disabled then there's no need for WP_ALL_ENABLE_MASK, i.e. a
non-zero generation would indicate write protection is enabled.  That'd
simplify the code and clean up the atomic usage.

>  }
>  
>  static void vmx_slot_disable_log_dirty(struct kvm *kvm,
> -- 
> 1.8.3.1
> 
>