Re: [PATCH 2/3] KVM: nVMX: Fix conditions for NMI and interrupt injection

Jan Kiszka
2013-03-14T15:41:24Z

On 2013-03-14 16:37, Gleb Natapov wrote:
> On Thu, Mar 14, 2013 at 04:24:18PM +0100, Jan Kiszka wrote:
>> On 2013-03-14 16:12, Gleb Natapov wrote:
>>> On Wed, Mar 13, 2013 at 05:53:45PM +0100, Jan Kiszka wrote:
>>>> If we are in guest mode, L0 can only inject events into L2 if L1 has
>>>> nothing pending. Otherwise, L0 would overwrite L1's events and they
>>>> would get lost. This check is conceptually independent of
>>>> nested_exit_on_intr.
>>>>
>>>> If L1 traps external interrupts, then we also need to look at L1's
>>>> idt_vectoring_info_field. If it is empty, we can kick the guest from L2
>>>> to L1, just like the previous code worked.
>>>>
>>>> Finally, the logic for checking interrupt has to be applied also on NMIs
>>>> in an analogous way. This enables NMI interception for nested guests.
>>>>
>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>> ---
>>>> arch/x86/kvm/vmx.c | 59 ++++++++++++++++++++++++++++++++++++++++++++-------
>>>> 1 files changed, 51 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>> index b50174d..10de336 100644
>>>> --- a/arch/x86/kvm/vmx.c
>>>> +++ b/arch/x86/kvm/vmx.c
>>>> @@ -4211,6 +4211,12 @@ static bool nested_exit_on_intr(struct kvm_vcpu *vcpu)
>>>> PIN_BASED_EXT_INTR_MASK;
>>>> }
>>>>
>>>> +static bool nested_exit_on_nmi(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + return get_vmcs12(vcpu)->pin_based_vm_exec_control &
>>>> + PIN_BASED_NMI_EXITING;
>>>> +}
>>>> +
>>>> static void enable_irq_window(struct kvm_vcpu *vcpu)
>>>> {
>>>> u32 cpu_based_vm_exec_control;
>>>> @@ -4307,6 +4313,30 @@ static void vmx_inject_nmi(struct kvm_vcpu *vcpu)
>>>>
>>>> static int vmx_nmi_allowed(struct kvm_vcpu *vcpu)
>>>> {
>>>> + if (is_guest_mode(vcpu)) {
>>>> + struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>>>> +
>>>> + if (to_vmx(vcpu)->nested.nested_run_pending &&
>>>> + (vmcs12->vm_entry_intr_info_field & INTR_INFO_VALID_MASK))
>>>> + return 0;
>>>> + if (nested_exit_on_nmi(vcpu)) {
>>>> + /*
>>>> + * Check if the idt_vectoring_info_field is free. We
>>>> + * cannot raise EXIT_REASON_EXCEPTION_NMI if it isn't.
>>>> + */
>>>> + if (vmcs12->idt_vectoring_info_field &
>>>> + VECTORING_INFO_VALID_MASK)
>>>> + return 0;
>>>> + nested_vmx_vmexit(vcpu);
>>>> + vmcs12->vm_exit_reason = EXIT_REASON_EXCEPTION_NMI;
>>>> + vmcs12->vm_exit_intr_info = NMI_VECTOR |
>>>> + INTR_TYPE_NMI_INTR | INTR_INFO_VALID_MASK;
>>>> + /*
>>>> + * fall through to normal code, but now in L1, not L2
>>>> + */
>>>> + }
>>>> + }
>>>> +
>>>> if (!cpu_has_virtual_nmis() && to_vmx(vcpu)->soft_vnmi_blocked)
>>>> return 0;
>>>>
>>>> @@ -4346,16 +4376,29 @@ static void vmx_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked)
>>>>
>>>> static int vmx_interrupt_allowed(struct kvm_vcpu *vcpu)
>>>> {
>>>> - if (is_guest_mode(vcpu) && nested_exit_on_intr(vcpu)) {
>>>> + if (is_guest_mode(vcpu)) {
>>>> struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>>>> - if (to_vmx(vcpu)->nested.nested_run_pending ||
>>>> - (vmcs12->idt_vectoring_info_field &
>>>> - VECTORING_INFO_VALID_MASK))
>>>> +
>>>> + if (to_vmx(vcpu)->nested.nested_run_pending &&
>>>> + (vmcs12->vm_entry_intr_info_field & INTR_INFO_VALID_MASK))
>>>> return 0;
>>> I do not understand this. As far as I remember Nadav's explanation we
>>> have to enter guest if nested_run_pending is set because VMX does not
>>> expect vmexit to happen without running guest at all. May be
>>> idt_vectoring_info_field processing is the only reason, may be not. I
>>> wouldn't gamble on it.
>>
>> What are the problems? Specifically if we emulate the effects of an
>> immediate vmexit properly. I'm not categorically excluding that some
>> case is missing. If we do not allow soft-vmexit here, we will set the
>> interrupt window later and will get such an immediate exit as well
>> (provided the L2 was interruptible).
>>
> Don't know. Some field that VMX change on vmresume and since from L2
> point of view vmresume was executed, but in reality it was not the
> result that L2 will see will be incorrect. vm_entry_intr_info_field is
> on of such fields, may be there are or will be more (vAPIC+posted
> interrupt)?

OK, better safe than sorry. Easy to change.

>
>>>
>>>> - nested_vmx_vmexit(vcpu);
>>>> - vmcs12->vm_exit_reason = EXIT_REASON_EXTERNAL_INTERRUPT;
>>>> - vmcs12->vm_exit_intr_info = 0;
>>>> - /* fall through to normal code, but now in L1, not L2 */
>>>> + if (nested_exit_on_intr(vcpu)) {
>>>> + /*
>>>> + * Check if the idt_vectoring_info_field is free. We
>>>> + * cannot raise EXIT_REASON_EXTERNAL_INTERRUPT if it
>>>> + * isn't.
>>>> + */
>>>> + if (vmcs12->idt_vectoring_info_field &
>>>> + VECTORING_INFO_VALID_MASK)
>>>> + return 0;
>>> I think we actually need to return 0 if idt_vectoring_info_field is
>>> valid even if !nested_exit_on_intr(). If we do not we let L0 inject
>>> interrupt into L2 and then overwrite it on entry from
>>> vmcs12->idt_vectoring_info_field.
>>
>> Sorry, don't understand the last sentence. This check is about the
>> software idt_vectoring_info_field, the one we keep for L1, not the real
>> field.
>>
>
> Suppose the vmcs12->idt_vectoring_info_field is valid and L0 want to
> inject an interrupt directly into L2 and L2 does not block interrupts.
> vmx_interrupt_allowed() will return true, so vmx_inject_irq()
> will be called and L0->L2 interrupt information will be written
> to VM_ENTRY_INTR_INFO_FIELD. Now in vmx_vcpu_run() there is again
> check for valid vmcs12->idt_vectoring_info_field and if it is valid
> VM_ENTRY_INTR_INFO_FIELD is overwritten with it. Interrupt that L0 just
> injected into L2 was lost forever.

OK, that's my fault in trying to split patch 2 and 3. Patch 3 will
remove that bogus overwriting from vmx_vcpu_run. I'll merge both patches
in the next round.

Jan

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