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

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 08, 2023, Santosh Shukla wrote:
> On 2/1/2023 5:36 AM, Sean Christopherson wrote:
> > On Tue, Jan 31, 2023, Sean Christopherson wrote:
> >> On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> >>> @@ -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.
> > 
> > Scratch that, dropping the second NMI in this case is correct.  The "running" part
> > of the existing "x86 is limited to one NMI running, and one NMI pending after it"
> > confused me.  The "running" thing is really just a variant on NMIs being blocked.
> > 
> > I'd also like to avoid the double decrement logic.  Accounting the virtual NMI is
> > a very different thing than dealing with concurrent NMIs, I'd prefer to reflect
> > that in the code.
> > 
> > Any objection to folding in the below to end up with:
> > 
> > 	unsigned limit;
> > 
> > 	/*
> > 	 * x86 is limited to one NMI pending, but because KVM can't react to
> > 	 * incoming NMIs as quickly as bare metal, e.g. if the vCPU is
> > 	 * scheduled out, KVM needs to play nice with two queued NMIs showing
> > 	 * up at the same time.  To handle this scenario, allow two NMIs to be
> > 	 * (temporarily) pending so long as NMIs are not blocked and KVM is not
> > 	 * waiting for a previous NMI injection to complete (which effectively
> > 	 * blocks NMIs).  KVM will immediately inject one of the two NMIs, and
> > 	 * will request an NMI window to handle the second NMI.
> > 	 */
> > 	if (static_call(kvm_x86_get_nmi_mask)(vcpu) || vcpu->arch.nmi_injected)
> > 		limit = 1;
> > 	else
> > 		limit = 2;
> > 
> > 	/*
> > 	 * Adjust the limit to account for pending virtual NMIs, which aren't
> > 	 * tracked in in vcpu->arch.nmi_pending.
> > 	 */
> > 	if (static_call(kvm_x86_is_vnmi_pending)(vcpu))
> > 		limit--;
> > 
> > 	vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
> > 	vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
> > 
> 
> I believe, you missed the function below hunk -
> 
> 	if (vcpu->arch.nmi_pending &&
> 	    static_call(kvm_x86_set_vnmi_pending(vcpu)))
> 		vcpu->arch.nmi_pending--;
> 
> Or am I missing something.. please suggest.

You're not missing anything, I'm pretty sure I just lost tracking of things.