本文将结合virtio spec与linux 源码，深入解析下virtio中的kick操作。

Overview

本文将详细的阐述virtio中的kick操作。根据kick操作的发展历史，按照如下顺序去介绍:

  1. legacy device kick
  2. modern device kick
  3. VIRTIO_F_NOTIFICATION_DATA feature的kick

legacy device

1
2
3
4
5
6
7
8
/* the notify function used when creating a virt queue */
bool vp_notify(struct virtqueue *vq)
{
	/* we write the queue's selector into the notification register to
	 * signal the other end */
	iowrite16(vq->index, (void __iomem *)vq->priv);
	return true;
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
// legacy device
static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
				  struct virtio_pci_vq_info *info,
				  unsigned int index,
				  void (*callback)(struct virtqueue *vq),
				  const char *name,
				  bool ctx,
				  u16 msix_vec)
{
        ...
	/* create the vring */
	vq = vring_create_virtqueue(index, num,
				    VIRTIO_PCI_VRING_ALIGN, &vp_dev->vdev,
				    true, false, ctx,
				    vp_notify, callback, name);
        ...
        vq->priv = (void __force *)vp_dev->ldev.ioaddr + VIRTIO_PCI_QUEUE_NOTIFY;
        ...
}

kick寄存器位于bar0中的VIRTIO_PCI_QUEUE_NOTIFY位置；不同vq使用同一个kick寄存器地址，往kick寄存器写入vq的index，告诉virtio后端要处理哪个vq。

modern device

在设备实现中，一般会将queue_notify_off设置为vq index；也就是说，vp_modern_get_queue_notify_off的返回值，一般会与输入变量index相同。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
/*
 * vp_modern_get_queue_notify_off - get notification offset for a virtqueue
 * @mdev: the modern virtio-pci device
 * @index: the queue index
 *
 * Returns the notification offset for a virtqueue
 */
static u16 vp_modern_get_queue_notify_off(struct virtio_pci_modern_device *mdev,
					  u16 index)
{
	vp_iowrite16(index, &mdev->common->queue_select);

	return vp_ioread16(&mdev->common->queue_notify_off);
}

  • 当notify_off_multiplier不为0时，不同vq使用不同的kick寄存器地址，往kick寄存器写入vq的index，告诉virtio后端要处理哪个vq
  • 当notify_off_multiplier为0时，不同vq使用相同的kick寄存器地址，往kick寄存器写入vq的index，告诉virtio后端要处理哪个vq
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
// modern device
static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
				  struct virtio_pci_vq_info *info,
				  unsigned int index,
				  void (*callback)(struct virtqueue *vq),
				  const char *name,
				  bool ctx,
				  u16 msix_vec)
{
        ...
	/* create the vring */
	vq = vring_create_virtqueue(index, num,
				    SMP_CACHE_BYTES, &vp_dev->vdev,
				    true, true, ctx,
				    notify, callback, name);
        ...
        vq->priv = (void __force *)vp_modern_map_vq_notify(mdev, index, NULL);
        ...
}

/*
 * vp_modern_map_vq_notify - map notification area for a
 * specific virtqueue
 * @mdev: the modern virtio-pci device
 * @index: the queue index
 * @pa: the pointer to the physical address of the nofity area
 *
 * Returns the address of the notification area
 */
void __iomem *vp_modern_map_vq_notify(struct virtio_pci_modern_device *mdev,
				      u16 index, resource_size_t *pa)
{
	u16 off = vp_modern_get_queue_notify_off(mdev, index);

	if (mdev->notify_base) {
		/* offset should not wrap */
		if ((u64)off * mdev->notify_offset_multiplier + 2
			> mdev->notify_len) {
			dev_warn(&mdev->pci_dev->dev,
				 "bad notification offset %u (x %u) "
				 "for queue %u > %zd",
				 off, mdev->notify_offset_multiplier,
				 index, mdev->notify_len);
			return NULL;
		}
		if (pa)
			*pa = mdev->notify_pa +
			      off * mdev->notify_offset_multiplier;
		return mdev->notify_base + off * mdev->notify_offset_multiplier;
	} else {
		...
	}
}

VIRTIO_F_NOTIFICATION_DATA feature

值得注意的是，对于split vq，desc table的最大size为2^16；对于packed vq，desc table的最大size为2^15；

在kick寄存器中，不止存放了vq index:

  • 对于split vq，kick寄存器中还存放了avail_idx
  • 对于packed vq，kick寄存器中还存放了avail_idx(为了表述的方便，严格来说，packed vq已经没有了avail ring，也就不存在avail_idx了)与wrap counter

vq_notif_config_data在一般情况下，就是vq index。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
// modern device
static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
				  struct virtio_pci_vq_info *info,
				  unsigned int index,
				  void (*callback)(struct virtqueue *vq),
				  const char *name,
				  bool ctx,
				  u16 msix_vec)
{
        ...
	if (__virtio_test_bit(&vp_dev->vdev, VIRTIO_F_NOTIFICATION_DATA))
		notify = vp_notify_with_data;
	else
		notify = vp_notify;
        ...
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
static bool vp_notify_with_data(struct virtqueue *vq)
{
	u32 data = vring_notification_data(vq);

	iowrite32(data, (void __iomem *)vq->priv);

	return true;
}

u32 vring_notification_data(struct virtqueue *_vq)
{
	struct vring_virtqueue *vq = to_vvq(_vq);
	u16 next;

	if (vq->packed_ring)
		next = (vq->packed.next_avail_idx &
				~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR))) |
			vq->packed.avail_wrap_counter <<
				VRING_PACKED_EVENT_F_WRAP_CTR;
	else
		next = vq->split.avail_idx_shadow;

	return next << 16 | _vq->index;
}

参考资料:

  1. virtio 0.9.5 spec
  2. virtio 1.3 spec