KVM: x86: Trigger I/O APIC route rescan in kvm_arch_irq_routing_update()

Trigger the I/O APIC route rescan that's performed for a split IRQ chip
after userspace updates IRQ routes in kvm_arch_irq_routing_update(), i.e.
before dropping kvm->irq_lock. Calling kvm_make_all_cpus_request() under
a mutex is perfectly safe, and the smp_wmb()+smp_mb__after_atomic() pair
in __kvm_make_request()+kvm_check_request() ensures the new routing is
visible to vCPUs prior to the request being visible to vCPUs.

In all likelihood, commit b053b2aef25d ("KVM: x86: Add EOI exit bitmap
inference") somewhat arbitrarily made the request outside of irq_lock to
avoid holding irq_lock any longer than is strictly necessary. And then
commit abdb080f7ac8 ("kvm/irqchip: kvm_arch_irq_routing_update renaming
split") took the easy route of adding another arch hook instead of risking
a functional change.

Note, the call to synchronize_srcu_expedited() does NOT provide ordering
guarantees with respect to vCPUs scanning the new routing; as above, the
request infrastructure provides the necessary ordering. I.e. there's no
need to wait for kvm_scan_ioapic_routes() to complete if it's actively
running, because regardless of whether it grabs the old or new table, the
vCPU will have another KVM_REQ_SCAN_IOAPIC pending, i.e. will rescan again
and see the new mappings.

Acked-by: Kai Huang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

arch/x86/kvm/irq_comm.c

···
 				   ARRAY_SIZE(default_routing), 0);
 }
 
-void kvm_arch_post_irq_routing_update(struct kvm *kvm)
-{
-	if (!irqchip_split(kvm))
-		return;
-	kvm_make_scan_ioapic_request(kvm);
-}
-
 void kvm_scan_ioapic_irq(struct kvm_vcpu *vcpu, u32 dest_id, u16 dest_mode,
 			 u8 vector, unsigned long *ioapic_handled_vectors)
 {
···
 #ifdef CONFIG_KVM_HYPERV
 	kvm_hv_irq_routing_update(kvm);
 #endif
+
+	if (irqchip_split(kvm))
+		kvm_make_scan_ioapic_request(kvm);
 }

include/linux/kvm_host.h

···
 
 #ifdef __KVM_HAVE_IOAPIC
 void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm);
-void kvm_arch_post_irq_routing_update(struct kvm *kvm);
 #else
 static inline void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm)
-{
-}
-static inline void kvm_arch_post_irq_routing_update(struct kvm *kvm)
 {
 }
 #endif

virt/kvm/irqchip.c

···
 	kvm_arch_irq_routing_update(kvm);
 	mutex_unlock(&kvm->irq_lock);
 
-	kvm_arch_post_irq_routing_update(kvm);
-
 	synchronize_srcu_expedited(&kvm->irq_srcu);
 
 	new = old;