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

Gleb Natapov
2013-03-14T15:37:55Z

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)?

> >
> >> - 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.

--
Gleb.