KVM: Intelligent device lookup on I/O bus

Currently the method of dealing with an IO operation on a bus (PIO/MMIO)
is to call the read or write callback for each device registered
on the bus until we find a device which handles it.

Since the number of devices on a bus can be significant due to ioeventfds
and coalesced MMIO zones, this leads to a lot of overhead on each IO
operation.

Instead of registering devices, we now register ranges which points to
a device. Lookup is done using an efficient bsearch instead of a linear
search.

Performance test was conducted by comparing exit count per second with
200 ioeventfds created on one byte and the guest is trying to access a
different byte continuously (triggering usermode exits).
Before the patch the guest has achieved 259k exits per second, after the
patch the guest does 274k exits per second.

Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/i8254.c

@@ -713,14 +713,16 @@ struct kvm_pit *kvm_create_pit(struct kvm *kvm, u32 flags)
 	kvm_register_irq_mask_notifier(kvm, 0, &pit->mask_notifier);
 
 	kvm_iodevice_init(&pit->dev, &pit_dev_ops);
-	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, &pit->dev);
+	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, KVM_PIT_BASE_ADDRESS,
+				      KVM_PIT_MEM_LENGTH, &pit->dev);
 	if (ret < 0)
 		goto fail;
 
 	if (flags & KVM_PIT_SPEAKER_DUMMY) {
 		kvm_iodevice_init(&pit->speaker_dev, &speaker_dev_ops);
 		ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS,
-						&pit->speaker_dev);
+					      KVM_SPEAKER_BASE_ADDRESS, 4,
+					      &pit->speaker_dev);
 		if (ret < 0)
 			goto fail_unregister;
 	}

arch/x86/kvm/i8259.c

@@ -459,15 +459,9 @@ static int picdev_in_range(gpa_t addr)
 	}
 }
 
-static inline struct kvm_pic *to_pic(struct kvm_io_device *dev)
+static int picdev_write(struct kvm_pic *s,
-{
-	return container_of(dev, struct kvm_pic, dev);
-}
-
-static int picdev_write(struct kvm_io_device *this,
 			 gpa_t addr, int len, const void *val)
 {
-	struct kvm_pic *s = to_pic(this);
 	unsigned char data = *(unsigned char *)val;
 	if (!picdev_in_range(addr))
 		return -EOPNOTSUPP;
@@ -494,10 +488,9 @@ static int picdev_write(struct kvm_io_device *this,
 	return 0;
 }
 
-static int picdev_read(struct kvm_io_device *this,
+static int picdev_read(struct kvm_pic *s,
 		       gpa_t addr, int len, void *val)
 {
-	struct kvm_pic *s = to_pic(this);
 	unsigned char data = 0;
 	if (!picdev_in_range(addr))
 		return -EOPNOTSUPP;
@@ -525,6 +518,48 @@ static int picdev_read(struct kvm_io_device *this,
 	return 0;
 }
 
+static int picdev_master_write(struct kvm_io_device *dev,
+			       gpa_t addr, int len, const void *val)
+{
+	return picdev_write(container_of(dev, struct kvm_pic, dev_master),
+			    addr, len, val);
+}
+
+static int picdev_master_read(struct kvm_io_device *dev,
+			      gpa_t addr, int len, void *val)
+{
+	return picdev_read(container_of(dev, struct kvm_pic, dev_master),
+			    addr, len, val);
+}
+
+static int picdev_slave_write(struct kvm_io_device *dev,
+			      gpa_t addr, int len, const void *val)
+{
+	return picdev_write(container_of(dev, struct kvm_pic, dev_slave),
+			    addr, len, val);
+}
+
+static int picdev_slave_read(struct kvm_io_device *dev,
+			     gpa_t addr, int len, void *val)
+{
+	return picdev_read(container_of(dev, struct kvm_pic, dev_slave),
+			    addr, len, val);
+}
+
+static int picdev_eclr_write(struct kvm_io_device *dev,
+			     gpa_t addr, int len, const void *val)
+{
+	return picdev_write(container_of(dev, struct kvm_pic, dev_eclr),
+			    addr, len, val);
+}
+
+static int picdev_eclr_read(struct kvm_io_device *dev,
+			    gpa_t addr, int len, void *val)
+{
+	return picdev_read(container_of(dev, struct kvm_pic, dev_eclr),
+			    addr, len, val);
+}
+
 /*
  * callback when PIC0 irq status changed
  */
@@ -537,9 +572,19 @@ static void pic_irq_request(struct kvm *kvm, int level)
 	s->output = level;
 }
 
-static const struct kvm_io_device_ops picdev_ops = {
+static const struct kvm_io_device_ops picdev_master_ops = {
-	.read     = picdev_read,
+	.read     = picdev_master_read,
-	.write    = picdev_write,
+	.write    = picdev_master_write,
+};
+
+static const struct kvm_io_device_ops picdev_slave_ops = {
+	.read     = picdev_slave_read,
+	.write    = picdev_slave_write,
+};
+
+static const struct kvm_io_device_ops picdev_eclr_ops = {
+	.read     = picdev_eclr_read,
+	.write    = picdev_eclr_write,
 };
 
 struct kvm_pic *kvm_create_pic(struct kvm *kvm)
@@ -560,16 +605,39 @@ struct kvm_pic *kvm_create_pic(struct kvm *kvm)
 	/*
 	 * Initialize PIO device
 	 */
-	kvm_iodevice_init(&s->dev, &picdev_ops);
+	kvm_iodevice_init(&s->dev_master, &picdev_master_ops);
+	kvm_iodevice_init(&s->dev_slave, &picdev_slave_ops);
+	kvm_iodevice_init(&s->dev_eclr, &picdev_eclr_ops);
 	mutex_lock(&kvm->slots_lock);
-	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, &s->dev);
+	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, 0x20, 2,
+				      &s->dev_master);
+	if (ret < 0)
+		goto fail_unlock;
+
+	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, 0xa0, 2, &s->dev_slave);
+	if (ret < 0)
+		goto fail_unreg_2;
+
+	ret = kvm_io_bus_register_dev(kvm, KVM_PIO_BUS, 0x4d0, 2, &s->dev_eclr);
+	if (ret < 0)
+		goto fail_unreg_1;
+
 	mutex_unlock(&kvm->slots_lock);
-	if (ret < 0) {
-		kfree(s);
-		return NULL;
-	}
 
 	return s;
+
+fail_unreg_1:
+	kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &s->dev_slave);
+
+fail_unreg_2:
+	kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &s->dev_master);
+
+fail_unlock:
+	mutex_unlock(&kvm->slots_lock);
+
+	kfree(s);
+
+	return NULL;
 }
 
 void kvm_destroy_pic(struct kvm *kvm)
@@ -577,7 +645,9 @@ void kvm_destroy_pic(struct kvm *kvm)
 	struct kvm_pic *vpic = kvm->arch.vpic;
 
 	if (vpic) {
-		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev);
+		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev_master);
+		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev_slave);
+		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &vpic->dev_eclr);
 		kvm->arch.vpic = NULL;
 		kfree(vpic);
 	}

arch/x86/kvm/irq.h

@@ -66,7 +66,9 @@ struct kvm_pic {
 	struct kvm *kvm;
 	struct kvm_kpic_state pics[2]; /* 0 is master pic, 1 is slave pic */
 	int output;		/* intr from master PIC */
-	struct kvm_io_device dev;
+	struct kvm_io_device dev_master;
+	struct kvm_io_device dev_slave;
+	struct kvm_io_device dev_eclr;
 	void (*ack_notifier)(void *opaque, int irq);
 	unsigned long irq_states[16];
 };

arch/x86/kvm/x86.c

@@ -3562,7 +3562,11 @@ long kvm_arch_vm_ioctl(struct file *filp,
 			if (r) {
 				mutex_lock(&kvm->slots_lock);
 				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
-							  &vpic->dev);
+							  &vpic->dev_master);
+				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
+							  &vpic->dev_slave);
+				kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
+							  &vpic->dev_eclr);
 				mutex_unlock(&kvm->slots_lock);
 				kfree(vpic);
 				goto create_irqchip_unlock;

include/linux/kvm_host.h

@@ -55,16 +55,16 @@ struct kvm;
 struct kvm_vcpu;
 extern struct kmem_cache *kvm_vcpu_cache;
 
-/*
+struct kvm_io_range {
- * It would be nice to use something smarter than a linear search, TBD...
+	gpa_t addr;
- * Thankfully we dont expect many devices to register (famous last words :),
+	int len;
- * so until then it will suffice.  At least its abstracted so we can change
+	struct kvm_io_device *dev;
- * in one place.
+};
- */
+
 struct kvm_io_bus {
 	int                   dev_count;
 #define NR_IOBUS_DEVS 300
-	struct kvm_io_device *devs[NR_IOBUS_DEVS];
+	struct kvm_io_range range[NR_IOBUS_DEVS];
 };
 
 enum kvm_bus {
@@ -77,8 +77,8 @@ int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 		     int len, const void *val);
 int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr, int len,
 		    void *val);
-int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx,
+int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
-			    struct kvm_io_device *dev);
+			    int len, struct kvm_io_device *dev);
 int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 			      struct kvm_io_device *dev);
 

virt/kvm/coalesced_mmio.c

@@ -141,7 +141,8 @@ int kvm_vm_ioctl_register_coalesced_mmio(struct kvm *kvm,
 	dev->zone = *zone;
 
 	mutex_lock(&kvm->slots_lock);
-	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, &dev->dev);
+	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, zone->addr,
+				      zone->size, &dev->dev);
 	if (ret < 0)
 		goto out_free_dev;
 	list_add_tail(&dev->list, &kvm->coalesced_zones);

virt/kvm/eventfd.c

@@ -586,7 +586,8 @@ kvm_assign_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
 
 	kvm_iodevice_init(&p->dev, &ioeventfd_ops);
 
-	ret = kvm_io_bus_register_dev(kvm, bus_idx, &p->dev);
+	ret = kvm_io_bus_register_dev(kvm, bus_idx, p->addr, p->length,
+				      &p->dev);
 	if (ret < 0)
 		goto unlock_fail;
 

virt/kvm/ioapic.c

@@ -394,7 +394,8 @@ int kvm_ioapic_init(struct kvm *kvm)
 	kvm_iodevice_init(&ioapic->dev, &ioapic_mmio_ops);
 	ioapic->kvm = kvm;
 	mutex_lock(&kvm->slots_lock);
-	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, &ioapic->dev);
+	ret = kvm_io_bus_register_dev(kvm, KVM_MMIO_BUS, ioapic->base_address,
+				      IOAPIC_MEM_LENGTH, &ioapic->dev);
 	mutex_unlock(&kvm->slots_lock);
 	if (ret < 0) {
 		kvm->arch.vioapic = NULL;

virt/kvm/kvm_main.c

@@ -47,6 +47,8 @@
 #include <linux/srcu.h>
 #include <linux/hugetlb.h>
 #include <linux/slab.h>
+#include <linux/sort.h>
+#include <linux/bsearch.h>
 
 #include <asm/processor.h>
 #include <asm/io.h>
@@ -2391,24 +2393,92 @@ static void kvm_io_bus_destroy(struct kvm_io_bus *bus)
 	int i;
 
 	for (i = 0; i < bus->dev_count; i++) {
-		struct kvm_io_device *pos = bus->devs[i];
+		struct kvm_io_device *pos = bus->range[i].dev;
 
 		kvm_iodevice_destructor(pos);
 	}
 	kfree(bus);
 }
 
+int kvm_io_bus_sort_cmp(const void *p1, const void *p2)
+{
+	const struct kvm_io_range *r1 = p1;
+	const struct kvm_io_range *r2 = p2;
+
+	if (r1->addr < r2->addr)
+		return -1;
+	if (r1->addr + r1->len > r2->addr + r2->len)
+		return 1;
+	return 0;
+}
+
+int kvm_io_bus_insert_dev(struct kvm_io_bus *bus, struct kvm_io_device *dev,
+			  gpa_t addr, int len)
+{
+	if (bus->dev_count == NR_IOBUS_DEVS)
+		return -ENOSPC;
+
+	bus->range[bus->dev_count++] = (struct kvm_io_range) {
+		.addr = addr,
+		.len = len,
+		.dev = dev,
+	};
+
+	sort(bus->range, bus->dev_count, sizeof(struct kvm_io_range),
+		kvm_io_bus_sort_cmp, NULL);
+
+	return 0;
+}
+
+int kvm_io_bus_get_first_dev(struct kvm_io_bus *bus,
+			     gpa_t addr, int len)
+{
+	struct kvm_io_range *range, key;
+	int off;
+
+	key = (struct kvm_io_range) {
+		.addr = addr,
+		.len = len,
+	};
+
+	range = bsearch(&key, bus->range, bus->dev_count,
+			sizeof(struct kvm_io_range), kvm_io_bus_sort_cmp);
+	if (range == NULL)
+		return -ENOENT;
+
+	off = range - bus->range;
+
+	while (off > 0 && kvm_io_bus_sort_cmp(&key, &bus->range[off-1]) == 0)
+		off--;
+
+	return off;
+}
+
 /* kvm_io_bus_write - called under kvm->slots_lock */
 int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 		     int len, const void *val)
 {
-	int i;
+	int idx;
 	struct kvm_io_bus *bus;
+	struct kvm_io_range range;
+
+	range = (struct kvm_io_range) {
+		.addr = addr,
+		.len = len,
+	};
 
 	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
-	for (i = 0; i < bus->dev_count; i++)
+	idx = kvm_io_bus_get_first_dev(bus, addr, len);
-		if (!kvm_iodevice_write(bus->devs[i], addr, len, val))
+	if (idx < 0)
+		return -EOPNOTSUPP;
+
+	while (idx < bus->dev_count &&
+		kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
+		if (!kvm_iodevice_write(bus->range[idx].dev, addr, len, val))
 			return 0;
+		idx++;
+	}
+
 	return -EOPNOTSUPP;
 }
 
@@ -2416,19 +2486,33 @@ int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 		    int len, void *val)
 {
-	int i;
+	int idx;
 	struct kvm_io_bus *bus;
+	struct kvm_io_range range;
+
+	range = (struct kvm_io_range) {
+		.addr = addr,
+		.len = len,
+	};
 
 	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
-	for (i = 0; i < bus->dev_count; i++)
+	idx = kvm_io_bus_get_first_dev(bus, addr, len);
-		if (!kvm_iodevice_read(bus->devs[i], addr, len, val))
+	if (idx < 0)
+		return -EOPNOTSUPP;
+
+	while (idx < bus->dev_count &&
+		kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
+		if (!kvm_iodevice_read(bus->range[idx].dev, addr, len, val))
 			return 0;
+		idx++;
+	}
+
 	return -EOPNOTSUPP;
 }
 
 /* Caller must hold slots_lock. */
-int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx,
+int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
-			    struct kvm_io_device *dev)
+			    int len, struct kvm_io_device *dev)
 {
 	struct kvm_io_bus *new_bus, *bus;
 
@@ -2440,7 +2524,7 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 	if (!new_bus)
 		return -ENOMEM;
 	memcpy(new_bus, bus, sizeof(struct kvm_io_bus));
-	new_bus->devs[new_bus->dev_count++] = dev;
+	kvm_io_bus_insert_dev(new_bus, dev, addr, len);
 	rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
 	synchronize_srcu_expedited(&kvm->srcu);
 	kfree(bus);
@@ -2464,9 +2548,13 @@ int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
 
 	r = -ENOENT;
 	for (i = 0; i < new_bus->dev_count; i++)
-		if (new_bus->devs[i] == dev) {
+		if (new_bus->range[i].dev == dev) {
 			r = 0;
-			new_bus->devs[i] = new_bus->devs[--new_bus->dev_count];
+			new_bus->dev_count--;
+			new_bus->range[i] = new_bus->range[new_bus->dev_count];
+			sort(new_bus->range, new_bus->dev_count,
+			     sizeof(struct kvm_io_range),
+			     kvm_io_bus_sort_cmp, NULL);
 			break;
 		}