On Fri, Jul 21, 2017 at 1:39 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote: > Hi Jim, > 2017-07-21 3:16 GMT+08:00 Jim Mattson <jmattson@xxxxxxxxxx>: >> On Wed, Jul 19, 2017 at 7:31 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote: >>> Hi Jim, >>> 2017-07-19 2:47 GMT+08:00 Jim Mattson <jmattson@xxxxxxxxxx>: >>>> Why do we expect the VM_EXIT_INTR_INFO and EXIT_QUALIFICATION fields >>>> of the VMCS to have the correct values for the injected exception? >>> >>> Good point, I think we should synthesize VM_EXIT_INTR_INFO and >>> EXIT_QUALIFICATION manually, I will post a patch for it. Btw, how >>> about setting EXIT_QULIFICATION to vcpu->arch.cr2 for the page fault >>> exception and 0 for other exceptions? >> >> From the SDM, section 27.1: >> >> If an event causes a VM exit directly, it does not update > > I mentioned this in the patch description: > >> However, there is no guarantee the exit reason is exception currently, when there is an external interrupt occurred on host, maybe a time interrupt for host which should not be injected to guest, and somewhere queues an exception, then the function nested_vmx_check_exception() will be called and the vmexit emulation codes will try to emulate the "Acknowledge interrupt on exit" behavior, the warning is triggered. > > If you think the scenario is correct, then it should be an event > causes a VM exit indirectly. So if both the scenario which I mentioned > and "This function > assumes it is called with the exit reason in vmcs02 being a #PF > exception" can happen, then maybe we should figure out how to fix both > scenarios suitable. In the situation you describe, the #PF causes a synthesized VM-exit from L2 to L1 directly, not indirectly. From the SDM: An exception causes a VM exit directly if the bit corresponding to that exception is set in the exception bitmap. Hence, CR2 should not be set yet. > >> architectural state as it would have if it had it not caused the VM >> exit: >> - A debug exception does not update DR6, DR7.GD, or >> IA32_DEBUGCTL.LBR. (Information about the nature of the debug >> exception is saved in the exit qualification field.) >> - A page fault does not update CR2. (The linear address causing >> the page fault is saved in the exit-qualification field.) >> >> This means that vcpu->arch.cr2 should not be set at this point for a >> #PF injection (and vcpu->arch.dr6 should not be set at this point for >> a #DB injection). For all other exceptions, yes, the exit >> qualification should be cleared. >> >>> >>> Regards, >>> Wanpeng Li >>> >>>> >>>> On Mon, Jun 5, 2017 at 5:19 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote: >>>>> 2017-06-05 20:07 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>: >>>>>> >>>>>> >>>>>> On 03/06/2017 05:21, Wanpeng Li wrote: >>>>>>> Commit 0b6ac343fc (KVM: nVMX: Correct handling of exception injection) mentioned >>>>>>> that "KVM wants to inject page-faults which it got to the guest. This function >>>>>>> assumes it is called with the exit reason in vmcs02 being a #PF exception". >>>>>>> Commit e011c663 (KVM: nVMX: Check all exceptions for intercept during delivery to >>>>>>> L2) allows to check all exceptions for intercept during delivery to L2. However, >>>>>>> there is no guarantee the exit reason is exception currently, when there is an >>>>>>> external interrupt occurred on host, maybe a time interrupt for host which should >>>>>>> not be injected to guest, and somewhere queues an exception, then the function >>>>>>> nested_vmx_check_exception() will be called and the vmexit emulation codes will >>>>>>> try to emulate the "Acknowledge interrupt on exit" behavior, the warning is >>>>>>> triggered. >>>>>>> >>>>>>> This patch fixes it by confirming to inject exception to the guest when the exit >>>>>>> reason in vmcs02 is exception. >>>>>> >>>>>> I am confused. On one hand, the comment originally "this is the only >>>>>> case in which KVM injects a #PF when L2 is running", but I'm not sure >>>>>> it's true. For example, KVM could emulate a movs while running L2. If >>>>>> the source is MMIO and the destination is a missing page, the original >>>>>> failure could be an EPT misconfig, but the access to the destination >>>>>> would cause a #PF in the guest (could be a nice testcase for >>>>>> kvm-unit-tests, BTW :)). >>>>>> >>>>>> On the other hand, why would you reuse to_vmx(vcpu)->exit_reason in >>>>>> nested_vmx_check_exception? Would the following fix the bug: >>>>>> >>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >>>>>> index 9b4b5d6dcd34..ca5d2b93385c 100644 >>>>>> --- a/arch/x86/kvm/vmx.c >>>>>> +++ b/arch/x86/kvm/vmx.c >>>>>> @@ -2425,7 +2425,7 @@ static int nested_vmx_check_exception(struct >>>>>> kvm_vcpu *vcpu, unsigned nr) >>>>>> if (!(vmcs12->exception_bitmap & (1u << nr))) >>>>>> return 0; >> >> Here, we should consult vmcs12->page_fault_error_code_mask and >> vmcs12->page_fault_error_code_match when nr==PF_VECTOR, as in >> nested_vmx_is_page_fault_vmexit(). > > Yeah, it should be done for "This function assumes it is called with > the exit reason in vmcs02 being a #PF exception". > > Regards, > Wanpeng Li
