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@xxxxxxxxxx>
> ---
> 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
>
