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

[Santosh Shukla <santosh.shukla@amd.com>]

On 2/16/2023 5:52 AM, Sean Christopherson wrote:
> 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.

Sure, Make sense. Will include V_NMI_BLOCKING_MASK set/clear in svm_set_nmi_mask() in v3
and will soon share patches for review.

Thanks.