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

[Gavin Shan]

Hi Eric,

On 11/10/21 11:37 PM, Eric Auger wrote:
> 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?

I think the commit log is misleading. list_empty_careful() is more strict
than list_empty(). In this patch, we replace list_empty() with list_empty_careful().
I will correct the commit log in next respin like below:

   All checks on the completion queue is done by the newly added inline
   function where list_empty_careful() instead of list_empty() is used.
 
> > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > ---
> >   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?

Access to the completion queue is protected by spinlock. So the additional
check in list_empty_careful() to verify the head's prev/next are modified
on the fly shouldn't happen. It means they're same in our case.

> >   		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,
Gavin