KVM: SVM: fix interrupt injection (apic->isr_count always 0)

In commit b4eef9b36db4, we started to use hwapic_isr_update() != NULL
instead of kvm_apic_vid_enabled(vcpu->kvm). This didn't work because
SVM had it defined and "apicv" path in apic_{set,clear}_isr() does not
change apic->isr_count, because it should always be 1. The initial
value of apic->isr_count was based on kvm_apic_vid_enabled(vcpu->kvm),
which is always 0 for SVM, so KVM could have injected interrupts when it
shouldn't.

Fix it by implicitly setting SVM's hwapic_isr_update to NULL and make the
initial isr_count depend on hwapic_isr_update() for good measure.

Fixes: b4eef9b36db4 ("kvm: x86: vmx: NULL out hwapic_isr_update() in case of !enable_apicv")
Reported-and-tested-by: Borislav Petkov <[e-mail]>
Signed-off-by: Radim Krčmář <[e-mail]>
Signed-off-by: Marcelo Tosatti <[e-mail]>

arch/x86/kvm/lapic.c

···
 		apic_set_reg(apic, APIC_TMR + 0x10 * i, 0);
 	}
 	apic->irr_pending = kvm_apic_vid_enabled(vcpu->kvm);
-	apic->isr_count = kvm_apic_vid_enabled(vcpu->kvm);
+	apic->isr_count = kvm_x86_ops->hwapic_isr_update ? 1 : 0;
 	apic->highest_isr_cache = -1;
 	update_divide_count(apic);
 	atomic_set(&apic->lapic_timer.pending, 0);
···
 	update_divide_count(apic);
 	start_apic_timer(apic);
 	apic->irr_pending = true;
-	apic->isr_count = kvm_apic_vid_enabled(vcpu->kvm) ?
+	apic->isr_count = kvm_x86_ops->hwapic_isr_update ?
 				1 : count_vectors(apic->regs + APIC_ISR);
 	apic->highest_isr_cache = -1;
 	if (kvm_x86_ops->hwapic_irr_update)

arch/x86/kvm/svm.c

···
 	return;
 }
 
-static void svm_hwapic_isr_update(struct kvm *kvm, int isr)
-{
-	return;
-}
-
 static void svm_sync_pir_to_irr(struct kvm_vcpu *vcpu)
 {
 	return;
···
 	.set_virtual_x2apic_mode = svm_set_virtual_x2apic_mode,
 	.vm_has_apicv = svm_vm_has_apicv,
 	.load_eoi_exitmap = svm_load_eoi_exitmap,
-	.hwapic_isr_update = svm_hwapic_isr_update,
 	.sync_pir_to_irr = svm_sync_pir_to_irr,
 
 	.set_tss_addr = svm_set_tss_addr,