KVM: x86: Save/restore all NMIs when multiple NMIs are pending

Save all pending NMIs in KVM_GET_VCPU_EVENTS, and queue KVM_REQ_NMI if one
or more NMIs are pending after KVM_SET_VCPU_EVENTS in order to re-evaluate
pending NMIs with respect to NMI blocking.

KVM allows multiple NMIs to be pending in order to faithfully emulate bare
metal handling of simultaneous NMIs (on bare metal, truly simultaneous
NMIs are impossible, i.e. one will always arrive first and be consumed).
Support for simultaneous NMIs botched the save/restore though.  KVM only
saves one pending NMI, but allows userspace to restore 255 pending NMIs
as kvm_vcpu_events.nmi.pending is a u8, and KVM's internal state is stored
in an unsigned int.

Fixes: 7460fb4a ("KVM: Fix simultaneous NMIs")
Signed-off-by: Santosh Shukla <Santosh.Shukla@amd.com>
Link: https://lore.kernel.org/r/20230227084016.3368-8-santosh.shukla@amd.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -5125,7 +5125,7 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
 	events->interrupt.shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
 
 	events->nmi.injected = vcpu->arch.nmi_injected;
-	events->nmi.pending = vcpu->arch.nmi_pending != 0;
+	events->nmi.pending = vcpu->arch.nmi_pending;
 	events->nmi.masked = static_call(kvm_x86_get_nmi_mask)(vcpu);
 
 	/* events->sipi_vector is never valid when reporting to user space */
@@ -5212,8 +5212,11 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
 						events->interrupt.shadow);
 
 	vcpu->arch.nmi_injected = events->nmi.injected;
-	if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING)
+	if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING) {
 		vcpu->arch.nmi_pending = events->nmi.pending;
+		if (vcpu->arch.nmi_pending)
+			kvm_make_request(KVM_REQ_NMI, vcpu);
+	}
 	static_call(kvm_x86_set_nmi_mask)(vcpu, events->nmi.masked);
 
 	if (events->flags & KVM_VCPUEVENT_VALID_SIPI_VECTOR &&