linux-kernel - Re: [PATCH v4 02/15] KVM: async_pf: Add helper function to check completion queue

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Date:   Wed, 10 Nov 2021 16:37:03 +0100
From:   Eric Auger <eauger@...hat.com>
To:     Gavin Shan <gshan@...hat.com>, kvmarm@...ts.cs.columbia.edu
Cc:     kvm@...r.kernel.org, maz@...nel.org, linux-kernel@...r.kernel.org,
        shan.gavin@...il.com, Jonathan.Cameron@...wei.com,
        pbonzini@...hat.com, vkuznets@...hat.com, will@...nel.org
Subject: Re: [PATCH v4 02/15] KVM: async_pf: Add helper function to check
 completion queue

Hi Gavin,

On 8/15/21 2:59 AM, Gavin Shan wrote:
> This adds inline helper kvm_check_async_pf_completion_queue() to
> check if there are pending completion in the queue. The empty stub
> is also added on !CONFIG_KVM_ASYNC_PF so that the caller needn't
> consider if CONFIG_KVM_ASYNC_PF is enabled.
> 
> All checks on the completion queue is done by the newly added inline
> function since list_empty() and list_empty_careful() are interchangeable.
why is it interchangeable?

> 
> Signed-off-by: Gavin Shan <gshan@...hat.com>
> ---
>  arch/x86/kvm/x86.c       |  2 +-
>  include/linux/kvm_host.h | 10 ++++++++++
>  virt/kvm/async_pf.c      | 10 +++++-----
>  virt/kvm/kvm_main.c      |  4 +---
>  4 files changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e5d5c5ed7dd4..7f35d9324b99 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11591,7 +11591,7 @@ static inline bool kvm_guest_apic_has_interrupt(struct kvm_vcpu *vcpu)
>  
>  static inline bool kvm_vcpu_has_events(struct kvm_vcpu *vcpu)
>  {
> -	if (!list_empty_careful(&vcpu->async_pf.done))
> +	if (kvm_check_async_pf_completion_queue(vcpu))
>  		return true;
>  
>  	if (kvm_apic_has_events(vcpu))
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 85b61a456f1c..a5f990f6dc35 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -339,12 +339,22 @@ struct kvm_async_pf {
>  	bool				notpresent_injected;
>  };
>  
> +static inline bool kvm_check_async_pf_completion_queue(struct kvm_vcpu *vcpu)
> +{
> +	return !list_empty_careful(&vcpu->async_pf.done);
> +}
> +
>  void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu);
>  void kvm_check_async_pf_completion(struct kvm_vcpu *vcpu);
>  bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  			unsigned long hva, struct kvm_arch_async_pf *arch);
>  int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu);
>  #else
> +static inline bool kvm_check_async_pf_completion_queue(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  static inline void kvm_check_async_pf_completion(struct kvm_vcpu *vcpu) { }
>  #endif
>  
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index dd777688d14a..d145a61a046a 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -70,7 +70,7 @@ static void async_pf_execute(struct work_struct *work)
>  		kvm_arch_async_page_present(vcpu, apf);
>  
>  	spin_lock(&vcpu->async_pf.lock);
> -	first = list_empty(&vcpu->async_pf.done);
> +	first = !kvm_check_async_pf_completion_queue(vcpu);
>  	list_add_tail(&apf->link, &vcpu->async_pf.done);
>  	apf->vcpu = NULL;
>  	spin_unlock(&vcpu->async_pf.lock);
> @@ -122,7 +122,7 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>  		spin_lock(&vcpu->async_pf.lock);
>  	}
>  
> -	while (!list_empty(&vcpu->async_pf.done)) {
> +	while (kvm_check_async_pf_completion_queue(vcpu)) {
this is replaced by a stronger check. Please can you explain why is it
equivalent?
>  		struct kvm_async_pf *work =
>  			list_first_entry(&vcpu->async_pf.done,
>  					 typeof(*work), link);
> @@ -138,7 +138,7 @@ void kvm_check_async_pf_completion(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_async_pf *work;
>  
> -	while (!list_empty_careful(&vcpu->async_pf.done) &&
> +	while (kvm_check_async_pf_completion_queue(vcpu) &&
>  	      kvm_arch_can_dequeue_async_page_present(vcpu)) {
>  		spin_lock(&vcpu->async_pf.lock);
>  		work = list_first_entry(&vcpu->async_pf.done, typeof(*work),
> @@ -205,7 +205,7 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
>  	struct kvm_async_pf *work;
>  	bool first;
>  
> -	if (!list_empty_careful(&vcpu->async_pf.done))
> +	if (kvm_check_async_pf_completion_queue(vcpu))
>  		return 0;
>  
>  	work = kmem_cache_zalloc(async_pf_cache, GFP_ATOMIC);
> @@ -216,7 +216,7 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
>  	INIT_LIST_HEAD(&work->queue); /* for list_del to work */
>  
>  	spin_lock(&vcpu->async_pf.lock);
> -	first = list_empty(&vcpu->async_pf.done);
> +	first = !kvm_check_async_pf_completion_queue(vcpu);
>  	list_add_tail(&work->link, &vcpu->async_pf.done);
>  	spin_unlock(&vcpu->async_pf.lock);
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index b50dbe269f4b..8795503651b1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3282,10 +3282,8 @@ static bool vcpu_dy_runnable(struct kvm_vcpu *vcpu)
>  	if (kvm_arch_dy_runnable(vcpu))
>  		return true;
>  
> -#ifdef CONFIG_KVM_ASYNC_PF
> -	if (!list_empty_careful(&vcpu->async_pf.done))
> +	if (kvm_check_async_pf_completion_queue(vcpu))
>  		return true;
> -#endif
>  
>  	return false;
>  }
> 
Thanks

Eric