Re: [PATCH v2 07/11] KVM: x86: add a delayed hardware NMI injection interface

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 15, 2023, Sean Christopherson wrote:
> On Tue, Feb 14, 2023, Santosh Shukla wrote:
> > "
> > V_NMI_MASK: Indicates whether virtual NMIs are masked. The processor will set V_NMI_MASK
> > once it takes the virtual NMI. V_NMI_MASK is cleared when the guest successfully completes an
> > IRET instruction or #VMEXIT occurs while delivering the virtual NMI
> > "
> >
> > In my initial implementation I had changed V_NMI_MASK for the SMM scenario [1],
> > This is also not required as HW will save the V_NMI/V_NMI_MASK on 
> > SMM entry and restore them on RSM.
> > 
> > That said the svm_{get,set}_nmi_mask will look something like:

...

> >  static void svm_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked)
> >  {
> >         struct vcpu_svm *svm = to_svm(vcpu);
> > 
> > +       if (is_vnmi_enabled(svm))
> > +               return;
> > +
> >         if (masked) {
> >                 svm->nmi_masked = true;
> >                 svm_set_iret_intercept(svm);
> > 
> > is there any inputs on above approach?
> 
> What happens if software clears the "NMIs are blocked" flag?  If KVM can't clear
> the flag, then we've got problems.  E.g. if KVM emulates IRET or SMI+RSM.  And I
> I believe there are use cases that use KVM to snapshot and reload vCPU state,
> e.g. record+replay?, in which case KVM_SET_VCPU_EVENTS needs to be able to adjust
> NMI blocking too.

Actually, what am I thinking.  Any type of state save/restore will need to stuff
NMI blocking.  E.g. live migration of a VM that is handling an NMI (V_NMI_MASK=1)
_and_ has a pending NMI (V_NMI=1) absolutely needs to set V_NMI_MASK=1 on the dest,
otherwise the pending NMI will get serviced when the guest expects NMIs to be blocked.