Re: [PATCH] KVM: x86: clean up conditions for asynchronous page fault handling

["Radim Krčmář" <rkrcmar@redhat.com>]

2019-06-13 13:03+0200, Paolo Bonzini:
> Even when asynchronous page fault is disabled, KVM does not want to pause
> the host if a guest triggers a page fault; instead it will put it into
> an artificial HLT state that allows running other host processes while
> allowing interrupt delivery into the guest.
> 
> However, the way this feature is triggered is a bit confusing.
> First, it is not used for page faults while a nested guest is
> running: but this is not an issue since the artificial halt
> is completely invisible to the guest, either L1 or L2.  Second,
> it is used even if kvm_halt_in_guest() returns true; in this case,
> the guest probably should not pay the additional latency cost of the
> artificial halt, and thus we should handle the page fault in a
> completely synchronous way.

The same reasoning would apply to kvm_mwait_in_guest(), so I would
disable APF with it as well.

> By introducing a new function kvm_can_deliver_async_pf, this patch
> commonizes the code that chooses whether to deliver an async page fault
> (kvm_arch_async_page_not_present) and the code that chooses whether a
> page fault should be handled synchronously (kvm_can_do_async_pf).
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -9775,6 +9775,36 @@ static int apf_get_user(struct kvm_vcpu *vcpu, u32 *val)
> +bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu)
> +{
> +	if (unlikely(!lapic_in_kernel(vcpu) ||
> +		     kvm_event_needs_reinjection(vcpu) ||
> +		     vcpu->arch.exception.pending))
> +		return false;
> +
> +	if (kvm_hlt_in_guest(vcpu->kvm) && !kvm_can_deliver_async_pf(vcpu))
> +		return false;
> +
> +	/*
> +	 * If interrupts are off we cannot even use an artificial
> +	 * halt state.

Can't we?  The artificial halt state would be canceled by the host page
fault handler.

> +	 */
> +	return kvm_x86_ops->interrupt_allowed(vcpu);
> @@ -9783,19 +9813,26 @@ void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
>  	trace_kvm_async_pf_not_present(work->arch.token, work->gva);
>  	kvm_add_async_pf_gfn(vcpu, work->arch.gfn);
>  
> -	if (!(vcpu->arch.apf.msr_val & KVM_ASYNC_PF_ENABLED) ||
> -	    (vcpu->arch.apf.send_user_only &&
> -	     kvm_x86_ops->get_cpl(vcpu) == 0))
> +	if (!kvm_can_deliver_async_pf(vcpu) ||
> +	    apf_put_user(vcpu, KVM_PV_REASON_PAGE_NOT_PRESENT)) {
> +		/*
> +		 * It is not possible to deliver a paravirtualized asynchronous
> +		 * page fault, but putting the guest in an artificial halt state
> +		 * can be beneficial nevertheless: if an interrupt arrives, we
> +		 * can deliver it timely and perhaps the guest will schedule
> +		 * another process.  When the instruction that triggered a page
> +		 * fault is retried, hopefully the page will be ready in the host.
> +		 */
>  		kvm_make_request(KVM_REQ_APF_HALT, vcpu);

A return is missing here, to prevent the delivery of PV APF.
(I'd probably keep the if/else.)

Thanks.

> -	else if (!apf_put_user(vcpu, KVM_PV_REASON_PAGE_NOT_PRESENT)) {
> -		fault.vector = PF_VECTOR;
> -		fault.error_code_valid = true;
> -		fault.error_code = 0;
> -		fault.nested_page_fault = false;
> -		fault.address = work->arch.token;
> -		fault.async_page_fault = true;
> -		kvm_inject_page_fault(vcpu, &fault);

>  	}
> +
> +	fault.vector = PF_VECTOR;
> +	fault.error_code_valid = true;
> +	fault.error_code = 0;
> +	fault.nested_page_fault = false;
> +	fault.address = work->arch.token;
> +	fault.async_page_fault = true;
> +	kvm_inject_page_fault(vcpu, &fault);
>  }
>  
>  void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
> -- 
> 1.8.3.1
>