Re: [PATCH 10/12] KVM: MMU: allow locklessly access shadow pagetable out of vcpu thread

[Gleb Natapov]

On Tue, Jul 30, 2013 at 09:02:08PM +0800, Xiao Guangrong wrote:
> It is easy if the handler is in the vcpu context, in that case we can use
> walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end() that
> disable interrupt to stop shadow page be freed. But we are on the ioctl context
> and the paths we are optimizing for have heavy workload, disabling interrupt is
> not good for the system performance
> 
> We add a indicator into kvm struct (kvm->arch.rcu_free_shadow_page), then use
> call_rcu() to free the shadow page if that indicator is set. Set/Clear the
> indicator are protected by slot-lock, so it need not be atomic and does not
> hurt the performance and the scalability
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  6 +++++-
>  arch/x86/kvm/mmu.c              | 23 +++++++++++++++++++++++
>  arch/x86/kvm/mmu.h              | 22 ++++++++++++++++++++++
>  3 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 531f47c..dc842b6 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -226,7 +226,10 @@ struct kvm_mmu_page {
>  	/* The page is obsolete if mmu_valid_gen != kvm->arch.mmu_valid_gen.  */
>  	unsigned long mmu_valid_gen;
>  
> -	DECLARE_BITMAP(unsync_child_bitmap, 512);
> +	union {
> +		DECLARE_BITMAP(unsync_child_bitmap, 512);
> +		struct rcu_head rcu;
> +	};
>  
>  #ifdef CONFIG_X86_32
>  	/*
> @@ -545,6 +548,7 @@ struct kvm_arch {
>  	 */
>  	struct list_head active_mmu_pages;
>  	struct list_head zapped_obsolete_pages;
> +	bool rcu_free_shadow_page;
>  
>  	struct list_head assigned_dev_head;
>  	struct iommu_domain *iommu_domain;
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index f8fc0cc..7f3391f 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2322,6 +2322,22 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>  	return ret;
>  }
>  
> +static void free_pages_rcu(struct rcu_head *head)
> +{
> +	struct kvm_mmu_page *next, *sp;
> +
> +	sp = container_of(head, struct kvm_mmu_page, rcu);
> +	while (sp) {
> +		if (!list_empty(&sp->link))
> +			next = list_first_entry(&sp->link,
> +					      struct kvm_mmu_page, link);
> +		else
> +			next = NULL;
> +		kvm_mmu_free_page(sp);
So here we are calling kvm_mmu_free_page() without holding mmu lock, why
is it safe?

> +		sp = next;
> +	}
> +}
> +
>  static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>  				    struct list_head *invalid_list)
>  {
> @@ -2342,6 +2358,13 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>  	 */
>  	kvm_flush_remote_tlbs(kvm);
>  
> +	if (kvm->arch.rcu_free_shadow_page) {
> +		sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
> +		list_del_init(invalid_list);
> +		call_rcu(&sp->rcu, free_pages_rcu);
> +		return;
> +	}
> +
>  	list_for_each_entry_safe(sp, nsp, invalid_list, link) {
>  		WARN_ON(!sp->role.invalid || sp->root_count);
>  		kvm_mmu_free_page(sp);
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 5b59c57..85405f1 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -115,4 +115,26 @@ static inline bool permission_fault(struct kvm_mmu *mmu, unsigned pte_access,
>  }
>  
>  void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm);
> +
> +/*
> + * The caller should ensure that these two functions should be
> + * serially called.
> + */
> +static inline void kvm_mmu_rcu_free_page_begin(struct kvm *kvm)
> +{
> +	rcu_read_lock();
> +
> +	kvm->arch.rcu_free_shadow_page = true;
> +	/* Set the indicator before access shadow page. */
> +	smp_mb();
> +}
> +
> +static inline void kvm_mmu_rcu_free_page_end(struct kvm *kvm)
> +{
> +	/* Make sure that access shadow page has finished. */
> +	smp_mb();
> +	kvm->arch.rcu_free_shadow_page = false;
> +
> +	rcu_read_unlock();
> +}
>  #endif
> -- 
> 1.8.1.4

--
			Gleb.
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