配置详解

• num_queue 一个queue就是一个vring
• cmd_size 单个请求需要额外存储的最大大小

vblk->tag_set.cmd_size =
		sizeof(struct virtblk_req) +
		sizeof(struct scatterlist) * sg_elems;

• queue_depth 等于queue_size / 2 是块设备同时可以处理的I/O请求数量

/* Default queue sizing is to fill the ring. */
	if (!virtblk_queue_depth) {
		virtblk_queue_depth = vblk->vqs[0].vq->num_free;
		/* ... but without indirect descs, we use 2 descs per req */
		if (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
			virtblk_queue_depth /= 2;
	}

• VIRTIO_BLK_F_SEG_MAX

/* We need to know how many segments before we allocate. */
	err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SEG_MAX,
				   struct virtio_blk_config, seg_max,
				   &sg_elems);

/* We need at least one SG element, whatever they say. */
	if (err || !sg_elems)
		sg_elems = 1;

	/* We need an extra sg elements at head and tail. */
	sg_elems += 2;

vblk->sg_elems = sg_elems;

• queue_size决定了vring的大小，就是vring descriptor所在的内存位置，用来存放vring descriptor本身的。

static inline unsigned vring_size(unsigned int num, unsigned long align)
{
	return ((sizeof(struct vring_desc) * num + sizeof(__virtio16) * (3 + num)
		 + align - 1) & ~(align - 1))
		+ sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
}

/* TODO: allocate each queue chunk individually */
	for (; num && vring_size(num, vring_align) > PAGE_SIZE; num /= 2) {
		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
					  &dma_addr,
					  GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
		if (queue)
			break;
		if (!may_reduce_num)
			return NULL;
	}

• VIRTIO_BLK_F_BLK_SIZE
• VIRTIO_BLK_F_CONFIG_WCE
• VIRTIO_BLK_F_FLUSH
• VIRTIO_BLK_F_TOPOLOGY
• VIRTIO_BLK_F_WRITE_ZEROES
• VIRTIO_BLK_F_SECURE_ERASE
• VIRTIO_BLK_F_ZONED
• VIRTIO_BLK_F_DISCARD
• VIRTIO_BLK_F_RO
• VIRTIO_BLK_F_GEOMETRY