by Santosh Shukla

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

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.

2023-02-24 14:40:00

by Maxim Levitsky

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

On Sat, 2023-01-28 at 01:09 +0000, Sean Christopherson wrote:
> On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> > This patch adds two new vendor callbacks:
>
> No "this patch" please, just say what it does.
>
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 684a5519812fb2..46993ce61c92db 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -871,8 +871,13 @@ struct kvm_vcpu_arch {
> > u64 tsc_scaling_ratio; /* current scaling ratio */
> >
> > atomic_t nmi_queued; /* unprocessed asynchronous NMIs */
> > - unsigned nmi_pending; /* NMI queued after currently running handler */
> > +
> > + unsigned int nmi_pending; /*
> > + * NMI queued after currently running handler
> > + * (not including a hardware pending NMI (e.g vNMI))
> > + */
>
> Put the block comment above. I'd say collapse all of the comments about NMIs into
> a single big block comment.
>
> > bool nmi_injected; /* Trying to inject an NMI this entry */
> > +
> > bool smi_pending; /* SMI queued after currently running handler */
> > u8 handling_intr_from_guest;
> >
> > @@ -10015,13 +10022,34 @@ static void process_nmi(struct kvm_vcpu *vcpu)
> > * Otherwise, allow two (and we'll inject the first one immediately).
> > */
> > if (static_call(kvm_x86_get_nmi_mask)(vcpu) || vcpu->arch.nmi_injected)
> > - limit = 1;
> > + limit--;
> > +
> > + /* Also if there is already a NMI hardware queued to be injected,
> > + * decrease the limit again
> > + */
>
> /*
> * Block comment ...
> */
>
> > + if (static_call(kvm_x86_get_hw_nmi_pending)(vcpu))
>
> I'd prefer "is_hw_nmi_pending()" over "get", even if it means not pairing with
> "set". Though I think that's a good thing since they aren't perfect pairs.
>
> > + limit--;
> >
> > - vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
> > + if (limit <= 0)
> > + return;
> > +
> > + /* Attempt to use hardware NMI queueing */
> > + if (static_call(kvm_x86_set_hw_nmi_pending)(vcpu)) {
> > + limit--;
> > + nmi_to_queue--;
> > + }
> > +
> > + vcpu->arch.nmi_pending += nmi_to_queue;
> > vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
> > kvm_make_request(KVM_REQ_EVENT, vcpu);
> > }
> >
> > +/* Return total number of NMIs pending injection to the VM */
> > +int kvm_get_total_nmi_pending(struct kvm_vcpu *vcpu)
> > +{
> > + return vcpu->arch.nmi_pending + static_call(kvm_x86_get_hw_nmi_pending)(vcpu);
>
> Nothing cares about the total count, this can just be;

I wanted to have the interface to be a bit more generic so that in theory you could have
more that one hardware NMI pending. I don't care much about it.

Best regards,
Maxim Levitsky

>
>
> bool kvm_is_nmi_pending(struct kvm_vcpu *vcpu)
> {
> return vcpu->arch.nmi_pending ||
> static_call(kvm_x86_is_hw_nmi_pending)(vcpu);
> }
>
>
> > +}
> > +
> > void kvm_make_scan_ioapic_request_mask(struct kvm *kvm,
> > unsigned long *vcpu_bitmap)
> > {
> > --
> > 2.26.3
> >