Re: [PATCH 2/2] KVM: async_pf: Inject 'page ready' event only if 'page not present' was previously injected

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jun 10, 2020 at 07:55:32PM +0200, Vitaly Kuznetsov wrote:
> 'Page not present' event may or may not get injected depending on
> guest's state. If the event wasn't injected, there is no need to
> inject the corresponding 'page ready' event as the guest may get
> confused. E.g. Linux thinks that the corresponding 'page not present'
> event wasn't delivered *yet* and allocates a 'dummy entry' for it.
> This entry is never freed.
> 
> Note, 'wakeup all' events have no corresponding 'page not present'
> event and always get injected.
> 
> s390 seems to always be able to inject 'page not present', the
> change is effectively a nop.
> 
> Suggested-by: Vivek Goyal <vgoyal@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/s390/include/asm/kvm_host.h | 2 +-
>  arch/s390/kvm/kvm-s390.c         | 4 +++-
>  arch/x86/include/asm/kvm_host.h  | 2 +-
>  arch/x86/kvm/x86.c               | 7 +++++--
>  include/linux/kvm_host.h         | 1 +
>  virt/kvm/async_pf.c              | 2 +-
>  6 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 3d554887794e..cee3cb6455a2 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -978,7 +978,7 @@ bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu);
>  void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu,
>  			       struct kvm_async_pf *work);
>  
> -void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
> +bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  				     struct kvm_async_pf *work);

Hi Vitaly,

A minor nit. Using return code to figure out if exception was injected
or not is little odd. How about we pass a pointer instead as parameter
and kvm_arch_async_page_not_present() sets it to true if page not
present exception was injected. This probably will be easier to
read.

If for some reason you don't like above, atleats it warrants a comment
explaining what do 0 and 1 mean.

Otherwise both the patches look good to me. I tested and I can confirm
that now page ready events are not being delivered to guest if page
not present was not injected.

Thanks
Vivek

>  
>  void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 06bde4bad205..33fea4488ef3 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3923,11 +3923,13 @@ static void __kvm_inject_pfault_token(struct kvm_vcpu *vcpu, bool start_token,
>  	}
>  }
>  
> -void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
> +bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  				     struct kvm_async_pf *work)
>  {
>  	trace_kvm_s390_pfault_init(vcpu, work->arch.pfault_token);
>  	__kvm_inject_pfault_token(vcpu, true, work->arch.pfault_token);
> +
> +	return true;
>  }
>  
>  void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6e03c021956a..f54e7499fc6a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1660,7 +1660,7 @@ void kvm_make_scan_ioapic_request(struct kvm *kvm);
>  void kvm_make_scan_ioapic_request_mask(struct kvm *kvm,
>  				       unsigned long *vcpu_bitmap);
>  
> -void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
> +bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  				     struct kvm_async_pf *work);
>  void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>  				 struct kvm_async_pf *work);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 13d0b0fa1a7c..75e4c68e9586 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10513,7 +10513,7 @@ bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu)
>  	return kvm_arch_interrupt_allowed(vcpu);
>  }
>  
> -void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
> +bool kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  				     struct kvm_async_pf *work)
>  {
>  	struct x86_exception fault;
> @@ -10530,6 +10530,7 @@ void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  		fault.address = work->arch.token;
>  		fault.async_page_fault = true;
>  		kvm_inject_page_fault(vcpu, &fault);
> +		return true;
>  	} else {
>  		/*
>  		 * It is not possible to deliver a paravirtualized asynchronous
> @@ -10540,6 +10541,7 @@ void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  		 * fault is retried, hopefully the page will be ready in the host.
>  		 */
>  		kvm_make_request(KVM_REQ_APF_HALT, vcpu);
> +		return false;
>  	}
>  }
>  
> @@ -10557,7 +10559,8 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>  		kvm_del_async_pf_gfn(vcpu, work->arch.gfn);
>  	trace_kvm_async_pf_ready(work->arch.token, work->cr2_or_gpa);
>  
> -	if (kvm_pv_async_pf_enabled(vcpu) &&
> +	if ((work->wakeup_all || work->notpresent_injected) &&
> +	    kvm_pv_async_pf_enabled(vcpu) &&
>  	    !apf_put_user_ready(vcpu, work->arch.token)) {
>  		vcpu->arch.apf.pageready_pending = true;
>  		kvm_apic_set_irq(vcpu, &irq, NULL);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 802b9e2306f0..2456dc5338f8 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -206,6 +206,7 @@ struct kvm_async_pf {
>  	unsigned long addr;
>  	struct kvm_arch_async_pf arch;
>  	bool   wakeup_all;
> +	bool notpresent_injected;
>  };
>  
>  void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu);
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index ba080088da76..a36828fbf40a 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -189,7 +189,7 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  
>  	list_add_tail(&work->queue, &vcpu->async_pf.queue);
>  	vcpu->async_pf.queued++;
> -	kvm_arch_async_page_not_present(vcpu, work);
> +	work->notpresent_injected = kvm_arch_async_page_not_present(vcpu, work);
>  
>  	schedule_work(&work->work);
>  
> -- 
> 2.25.4
>