Re: [PATCHv3 5/8] KVM: SVM: Add VNMI support in inject_nmi

[Maciej S. Szmigiero]

On 25.08.2022 12:56, Shukla, Santosh wrote:
> On 8/24/2022 6:26 PM, Maciej S. Szmigiero wrote:
> > On 24.08.2022 14:13, Shukla, Santosh wrote:
> > > Hi Maciej,
> > >
> > > On 8/11/2022 2:54 AM, Maciej S. Szmigiero wrote:
> > > > On 10.08.2022 08:12, Santosh Shukla wrote:
> > > > > Inject the NMI by setting V_NMI in the VMCB interrupt control. processor
> > > > > will clear V_NMI to acknowledge processing has started and will keep the
> > > > > V_NMI_MASK set until the processor is done with processing the NMI event.
> > > > >
> > > > > Signed-off-by: Santosh Shukla <santosh.shukla@xxxxxxx>
> > > > > ---
> > > > > v3:
> > > > > - Removed WARN_ON check.
> > > > >
> > > > > v2:
> > > > > - Added WARN_ON check for vnmi pending.
> > > > > - use `get_vnmi_vmcb` to get correct vmcb so to inject vnmi.
> > > > >
> > > > >     arch/x86/kvm/svm/svm.c | 7 +++++++
> > > > >     1 file changed, 7 insertions(+)
> > > > >
> > > > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > > > index e260e8cb0c81..8c4098b8a63e 100644
> > > > > --- a/arch/x86/kvm/svm/svm.c
> > > > > +++ b/arch/x86/kvm/svm/svm.c
> > > > > @@ -3479,7 +3479,14 @@ static void pre_svm_run(struct kvm_vcpu *vcpu)
> > > > >     static void svm_inject_nmi(struct kvm_vcpu *vcpu)
> > > > >     {
> > > > >         struct vcpu_svm *svm = to_svm(vcpu);
> > > > > +    struct vmcb *vmcb = NULL;
> > > > >     +    if (is_vnmi_enabled(svm)) {
> > > >
> > > > I guess this should be "is_vnmi_enabled(svm) && !svm->nmi_l1_to_l2"
> > > > since if nmi_l1_to_l2 is true then the NMI to be injected originally
> > > > comes from L1's VMCB12 EVENTINJ field.
> > >
> > > Not sure if I understood the case fully.. so trying to sketch scenario here -
> > > if nmi_l1_to_l2 is true then event is coming from EVTINJ. .which could
> > > be one of following case -
> > > 1) L0 (vnmi enabled) and L1 (vnmi disabled)
> >
> > As far as I can see in this case:
> > is_vnmi_enabled() returns whether VMCB02's int_ctl has V_NMI_ENABLE bit set.
>
> For L1 with vnmi disabled case - is_vnmi_enabled()->get_vnmi_vmcb() will return false so the
> execution path will opt EVTINJ model for re-injection.

I guess by "get_vnmi_vmcb() will return false" you mean it will return NULL,
since this function returns a pointer, not a bool.

I can't see however, how this will happen:
> static inline struct vmcb *get_vnmi_vmcb(struct vcpu_svm *svm)
> {
> 	if (!vnmi)
> 		return NULL;
        ^ "vnmi" variable controls whether L0 uses vNMI,
	   so this variable is true in our case

> 	if (is_guest_mode(&svm->vcpu))
> 		return svm->nested.vmcb02.ptr;
		^ this should be always non-NULL.

So get_vnmi_vmcb() will return VMCB02 pointer in our case, not NULL...

> Thanks,
> Santosh
>
> > This field in VMCB02 comes from nested_vmcb02_prepare_control() which
> > in the !nested_vnmi_enabled() case (L1 is not using vNMI) copies these bits
> > from VMCB01:
> > > int_ctl_vmcb01_bits |= (V_NMI_PENDING | V_NMI_ENABLE | V_NMI_MASK);
> >
> > So in this case (L0 uses vNMI) V_NMI_ENABLE will be set in VMCB01, right?
> >
> > This bit will then be copied to VMCB02 

... and due to the above is_vnmi_enabled() will return true, so
re-injection will attempt to use vNMI instead of EVTINJ (wrong).

> > > 2) L0 & L1 both vnmi disabled.
> >
> > This case is ok.
> >
> > > In both cases the vnmi check will fail for L1 and execution path
> > > will fall back to default - right?
> > >
> > > Thanks,
> > > Santosh

Thanks,
Maciej