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

[Santosh Shukla]

On 2/1/2023 3:58 AM, Sean Christopherson wrote:
> On Tue, Nov 29, 2022, Maxim Levitsky wrote:
>> @@ -5191,9 +5191,12 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
>>  
>>  	vcpu->arch.nmi_injected = events->nmi.injected;
>>  	if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING)
>> -		vcpu->arch.nmi_pending = events->nmi.pending;
>> +		atomic_add(events->nmi.pending, &vcpu->arch.nmi_queued);
>> +
>>  	static_call(kvm_x86_set_nmi_mask)(vcpu, events->nmi.masked);
>>  
>> +	process_nmi(vcpu);
> 
> Argh, having two process_nmi() calls is ugly (not blaming your code, it's KVM's
> ABI that's ugly).  E.g. if we collapse this down, it becomes:
> 
> 	process_nmi(vcpu);
> 
> 	if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING) {
> 		<blah blah blah>
> 	}
> 	static_call(kvm_x86_set_nmi_mask)(vcpu, events->nmi.masked);
> 
> 	process_nmi(vcpu);
> 
> And the second mess is that V_NMI needs to be cleared.
> 

Can you please elaborate on "V_NMI cleared" scenario? Are you mentioning about V_NMI_MASK or svm->nmi_masked?

> The first process_nmi() effectively exists to (a) purge nmi_queued and (b) keep
> nmi_pending if KVM_VCPUEVENT_VALID_NMI_PENDING is not set.  I think we can just
> replace that with an set of nmi_queued, i.e.
> 
> 	if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING) {
> 		vcpu->arch-nmi_pending = 0;	
> 		atomic_set(&vcpu->arch.nmi_queued, events->nmi.pending);
> 		process_nmi();
> 
You mean replace above process_nmi() with kvm_make_request(KVM_REQ_NMI, vcpu), right?
I'll try with above proposal.

>	}
> 
> because if nmi_queued is non-zero in the !KVM_VCPUEVENT_VALID_NMI_PENDING, then
> there should already be a pending KVM_REQ_NMI.  Alternatively, replace process_nmi()
> with a KVM_REQ_NMI request (that probably has my vote?).
> 
> If that works, can you do that in a separate patch?  Then this patch can tack on
> a call to clear V_NMI.
> 
>>  	if (events->flags & KVM_VCPUEVENT_VALID_SIPI_VECTOR &&
>>  	    lapic_in_kernel(vcpu))
>>  		vcpu->arch.apic->sipi_vector = events->sipi_vector;
>> @@ -10008,6 +10011,10 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
>>  static void process_nmi(struct kvm_vcpu *vcpu)
>>  {
>>  	unsigned limit = 2;
>> +	int nmi_to_queue = atomic_xchg(&vcpu->arch.nmi_queued, 0);
>> +
>> +	if (!nmi_to_queue)
>> +		return;
>>  
>>  	/*
>>  	 * x86 is limited to one NMI running, and one NMI pending after it.
>> @@ -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
>> +	 */
>> +	if (static_call(kvm_x86_get_hw_nmi_pending)(vcpu))
>> +		limit--;
> 
> I don't think this is correct.  If a vNMI is pending and NMIs are blocked, then
> limit will end up '0' and KVM will fail to pend the additional NMI in software.
> After much fiddling, and factoring in the above, how about this?
> 
> 	unsigned limit = 2;
> 
> 	/*
> 	 * x86 is limited to one NMI running, and one NMI pending after it.
> 	 * If an NMI is already in progress, limit further NMIs to just one.
> 	 * Otherwise, allow two (and we'll inject the first one immediately).
> 	 */
> 	if (vcpu->arch.nmi_injected) {
> 		/* vNMI counts as the "one pending NMI". */
> 		if (static_call(kvm_x86_is_vnmi_pending)(vcpu))
> 			limit = 0;
> 		else
> 			limit = 1;
> 	} else if (static_call(kvm_x86_get_nmi_mask)(vcpu) ||
> 		   static_call(kvm_x86_is_vnmi_pending)(vcpu)) {
> 		limit = 1;
> 	}
> 
> 	vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
> 	vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
> 
> 	if (vcpu->arch.nmi_pending &&
> 	    static_call(kvm_x86_set_vnmi_pending(vcpu)))
> 		vcpu->arch.nmi_pending--;
> 
> 	if (vcpu->arch.nmi_pending)
> 		kvm_make_request(KVM_REQ_EVENT, vcpu);
> 
> With the KVM_REQ_EVENT change in a separate prep patch:
> 
> --
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Date: Tue, 31 Jan 2023 13:33:02 -0800
> Subject: [PATCH] KVM: x86: Raise an event request when processing NMIs iff an
>  NMI is pending
> 
> Don't raise KVM_REQ_EVENT if no NMIs are pending at the end of
> process_nmi().  Finishing process_nmi() without a pending NMI will become
> much more likely when KVM gains support for AMD's vNMI, which allows
> pending vNMIs in hardware, i.e. doesn't require explicit injection.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 508074e47bc0..030136b6ebbd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10134,7 +10134,9 @@ static void process_nmi(struct kvm_vcpu *vcpu)
>  
>  	vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
>  	vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
> -	kvm_make_request(KVM_REQ_EVENT, vcpu);
> +
> +	if (vcpu->arch.nmi_pending)
> +		kvm_make_request(KVM_REQ_EVENT, vcpu);
>  }
>  
>  void kvm_make_scan_ioapic_request_mask(struct kvm *kvm,
> 

Looks good to me, will include as separate patch.

Thanks,
Santosh