kvm: irqfd: avoid update unmodified entries of the routing

All of the irqfds would to be updated when update the irq
routing, it's too expensive if there're too many irqfds.

However we can reduce the cost by avoid some unnecessary
updates. For irqs of MSI type on X86, the update can be
saved if the msi values are not change.

The vfio migration could receives benefit from this optimi-
zaiton. The test VM has 128 vcpus and 8 VF (with 65 vectors
enabled), so the VM has more than 520 irqfds. We mesure the
cost of the vfio_msix_enable (in QEMU, it would set routing
for each irqfd) for each VF, and we can see the total cost
can be significantly reduced.

                Origin         Apply this Patch
1st             8              4
2nd             15             5
3rd             22             6
4th             24             6
5th             36             7
6th             44             7
7th             51             8
8th             58             8
Total           258ms          51ms

We're also tring to optimize the QEMU part [1], but it's still
worth to optimize the KVM to gain more benefits.

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg04215.html

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Message-Id: <20210827080003.2689-1-longpeng2@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -12062,6 +12062,15 @@ int kvm_arch_update_irqfd_routing(struct kvm *kvm, unsigned int host_irq,
 	return static_call(kvm_x86_update_pi_irte)(kvm, host_irq, guest_irq, set);
 }
 
+bool kvm_arch_irqfd_route_changed(struct kvm_kernel_irq_routing_entry *old,
+				  struct kvm_kernel_irq_routing_entry *new)
+{
+	if (new->type != KVM_IRQ_ROUTING_MSI)
+		return true;
+
+	return !!memcmp(&old->msi, &new->msi, sizeof(new->msi));
+}
+
 bool kvm_vector_hashing_enabled(void)
 {
 	return vector_hashing;

include/linux/kvm_host.h

@@ -1764,6 +1764,8 @@ void kvm_arch_irq_bypass_stop(struct irq_bypass_consumer *);
 void kvm_arch_irq_bypass_start(struct irq_bypass_consumer *);
 int kvm_arch_update_irqfd_routing(struct kvm *kvm, unsigned int host_irq,
 				  uint32_t guest_irq, bool set);
+bool kvm_arch_irqfd_route_changed(struct kvm_kernel_irq_routing_entry *,
+				  struct kvm_kernel_irq_routing_entry *);
 #endif /* CONFIG_HAVE_KVM_IRQ_BYPASS */
 
 #ifdef CONFIG_HAVE_KVM_INVALID_WAKEUPS

virt/kvm/eventfd.c

@@ -281,6 +281,13 @@ int  __attribute__((weak)) kvm_arch_update_irqfd_routing(
 {
 	return 0;
 }
+
+bool __attribute__((weak)) kvm_arch_irqfd_route_changed(
+				struct kvm_kernel_irq_routing_entry *old,
+				struct kvm_kernel_irq_routing_entry *new)
+{
+	return true;
+}
 #endif
 
 static int
@@ -615,10 +622,16 @@ void kvm_irq_routing_update(struct kvm *kvm)
 	spin_lock_irq(&kvm->irqfds.lock);
 
 	list_for_each_entry(irqfd, &kvm->irqfds.items, list) {
+#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
+		/* Under irqfds.lock, so can read irq_entry safely */
+		struct kvm_kernel_irq_routing_entry old = irqfd->irq_entry;
+#endif
+
 		irqfd_update(kvm, irqfd);
 
 #ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
-		if (irqfd->producer) {
+		if (irqfd->producer &&
+		    kvm_arch_irqfd_route_changed(&old, &irqfd->irq_entry)) {
 			int ret = kvm_arch_update_irqfd_routing(
 					irqfd->kvm, irqfd->producer->irq,
 					irqfd->gsi, 1);