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Message-ID: <2543ace0-444a-7777-460b-c3eab9eb612a@redhat.com>
Date: Thu, 13 Jan 2022 15:38:43 +0800
From: Gavin Shan <gshan@...hat.com>
To: Eric Auger <eauger@...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 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@...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?
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
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