> -----Original Message----- > From: Paolo Bonzini [mailto:pbonzini@xxxxxxxxxx] > Sent: Wednesday, September 28, 2016 4:22 PM > To: Michael S. Tsirkin <mst@xxxxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; > yang.zhang.wz@xxxxxxxxx; Wu, Feng <feng.wu@xxxxxxxxx>; > rkrcmar@xxxxxxxxxx > Subject: Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv > interrupt injection > > > > On 28/09/2016 01:07, Michael S. Tsirkin wrote: > > On Tue, Sep 27, 2016 at 11:20:12PM +0200, Paolo Bonzini wrote: > >> Since bf9f6ac8d749 ("KVM: Update Posted-Interrupts Descriptor when vCPU > >> is blocked", 2015-09-18) the posted interrupt descriptor is checked > >> unconditionally for PIR.ON. Therefore we don't need KVM_REQ_EVENT to > >> trigger the scan and, if NMIs or SMIs are not involved, we can avoid > >> the complicated event injection path. > >> > >> However, there is a race between vmx_deliver_posted_interrupt and > >> vcpu_enter_guest. Fix it by disabling interrupts before vcpu->mode is > >> set to IN_GUEST_MODE. > > > > Could you describe the race a bit more please? > > I'm surprised that locally disabling irqs > > fixes a race with a different CPUs. > > The posted interrupt IPI has a dummy handler in arch/x86/kernel/irq.c > (smp_kvm_posted_intr_ipi). It only does something if it is received > while the guest is running. > > So local_irq_disable has an interesting side effect. Because the > interrupt will not be processed until the guest is entered, > local_irq_disable effectively switches the IRQ handler from the dummy > handler to the processor's posted interrupt handling. > > So you want to do that before setting IN_GUEST_MODE, otherwise the IPI > sent by deliver_posted_interrupt is ignored. IIUIC, the issue you describe above is that IPI for posted-interrupts may be issued between vcpu->mode = IN_GUEST_MODE; and local_irq_disable(); But if that really happens, we will call kvm_vcpu_kick() in vmx_deliver_posted_interrupt(), hence the vcpu->mode will be changed to EXITING_GUEST_MODE, then we will goto cancel_injection in vcpu_enter_guest, so the posted-interrupt will be delivered to guest in the next vmentry. Seems I cannot see the problem. Do I miss something? Thanks, Feng > > However, the patch is wrong, because this bit: > > if (kvm_lapic_enabled(vcpu)) { > /* > * Update architecture specific hints for APIC > * virtual interrupt delivery. > */ > if (vcpu->arch.apicv_active) > kvm_x86_ops->hwapic_irr_update(vcpu, > kvm_lapic_find_highest_irr(vcpu)); > } > > also has to be moved after setting IN_GUEST_MODE. Basically the order > for interrupt injection is: > > (1) set PIR > smp_wmb() > (2) set ON > smp_mb() > (3) read vcpu->mode > if IN_GUEST_MODE > (4a) send posted interrupt IPI > else > (4b) kick (i.e. cmpxchg vcpu->mode from IN_GUEST_MODE to > EXITING_GUEST_MODE and send reschedule IPI) > > while the order for entering the guest must be the opposite. The > numbers on the left identify the pairing between interrupt injection and > vcpu_entr_guest > > (4a) enable posted interrupt processing (i.e. disable interrupts!) > (3) set vcpu->mode to IN_GUEST_MODE > smp_mb() > (2) read ON > if ON then > (1) read PIR > sync PIR to IRR > (4b) read vcpu->mode > if vcpu->mode == EXITING_GUEST_MODE then > cancel vmentry > (3/2/1) # posted interrupts are processed on the next vmentry > > Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
