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 <[email protected]> > --- > 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 >
