On Thu, May 14, 2009 at 09:43:33PM +0800, Dong, Eddie wrote: > Avi Kivity wrote: > > Dong, Eddie wrote: > >> OK. > >> Also back to Gleb's question, the reason I want to do that is to > >> simplify event > >> generation mechanism in current KVM. > >> > >> Today KVM use additional layer of exception/nmi/interrupt such as > >> vcpu.arch.exception.pending, vcpu->arch.interrupt.pending & > >> vcpu->arch.nmi_injected. > >> All those additional layer is due to compete of > >> VM_ENTRY_INTR_INFO_FIELD > >> write to inject the event. Both SVM & VMX has only one resource to > >> inject the virtual event but KVM generates 3 catagory of events in > >> parallel which further requires additional > >> logic to dictate among them. > > > > I thought of using a queue to hold all pending events (in a common > > format), sort it by priority, and inject the head. > > The SDM Table 5-4 requires to merge 2 events together, i.e. convert to #DF/ > Triple fault or inject serially when 2 events happens no matter NMI, IRQ or exception. > > As if considering above events merging activity, that is a single element queue. I don't know how you got to this conclusion from you previous statement. See explanation to table 5-2 for instate where it is stated that interrupt should be held pending if there is exception with higher priority. Should be held pending where? In the queue, like we do. Note that low prio exceptions are just dropped since they will be regenerated. > We could have either: 1) A pure SW "queue" that will be flush to HW > register later (VM_ENTRY_INTR_INFO_FIELD), 2) Direct use HW register. > We have three event sources 1) exceptions 2) IRQ 3) NMI. We should have queue of three elements sorted by priority. On each entry we should inject an event with highest priority. And remove it from queue on exit. > > A potential benefit is that it can avoid duplicated code and potential bugs > in current code as following patch shows if I understand correctly: > > --- a/arch/x86/kvm/vmx.c > +++ b/arch/x86/kvm/vmx.c > @@ -2599,7 +2599,7 @@ static int handle_exception(struct kvm_vcpu *vcpu, struct > kvm_run *kvm_run) > cr2 = vmcs_readl(EXIT_QUALIFICATION); > KVMTRACE_3D(PAGE_FAULT, vcpu, error_code, (u32)cr2, > (u32)((u64)cr2 >> 32), handler); > - if (vcpu->arch.interrupt.pending || vcpu->arch.exception.pending > ) > + if (vcpu->arch.interrupt.pending || vcpu->arch.exception.pending > || vcpu->arch.nmi_injected) > kvm_mmu_unprotect_page_virt(vcpu, cr2); > return kvm_mmu_page_fault(vcpu, cr2, error_code); > } This fix is already in Avi's tree (not yet pushed). > Either way are OK and up to you. BTW Xen uses HW register directly to representing > an pending event. > In this particular case I don't mind to use HW register either, but I don't see any advantage. > > > >> One example is that exception has higher priority > >> than NMI/IRQ injection in current code which is not true in reality. > >> > > > > I don't think it matters in practice, since the guest will see it as a > > timing issue. NMIs and IRQs are asynchronous (even those generated by > > the guest through the local APIC). > > Yes. But also cause IRQ injection be delayed which may have side effect. > For example if guest exception handler is very longer or if guest VCPU fall into > recursive #GP. Within current logic, a guest IRQ event from KDB (IPI) running > on VCPU0, as an example, can't force the dead loop VCPU1 into KDB since it > is recursively #GP. If one #GP causes another #GP this is a #DF. If CPU has a chance to executes something in between KVM will have a chance to inject NMI. > > > > >> Another issue is that an failed event from previous injection say > >> IRQ or NMI may be discarded if an virtual exception happens in the > >> EXIT handling now. With the patch of generic double fault handling, > >> this case should be handled as normally. > >> > > > > Discarding an exception is usually okay as it will be regenerated. I > > don't think we discard interrupts or NMIs. > In reality (Running OS in guest), it doesn't happen so far. But architecturally, > it could. For example KVM injects an IRQ, but VM Resume get #PF and > back to KVM with IDT_VECTORING valid. Then KVM will put back the failed > IRQ to interrupt queue. But if #PF handling generates another exception, > then the interrupt queue won't be able to be injected, since KVM inject > exception first. And the interrupt queue is discarded at next VM Exit. > I acknowledge the presence of the bug although I was not able to write a test case to cause it yet, but it is easy to fix this without changing code too much. Unified event queue and clearing of only injected event on exit should do the trick. -- Gleb. -- 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
