接前一篇文章： QEMU源码全解析 —— 块设备虚拟化（13）
本文内容参考：
《 QEMU /KVM源码解析与应用》 —— 李强，机械工业出版社
特此致谢！
QEMU初始化阶段的块设备虚拟化
上一回开始解析VirtioDeviceClass的realize函数virtio_blk_device_realize()，再来回顾一下。virtio_blk_device_realize函数在同文件（hw/block/virtio-blk.c）中，代码如下：
static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
{
VirtIODevice *vdev = VIRTIO_DEVICE(dev);
VirtIOBlock *s = VIRTIO_BLK(dev);
VirtIOBlkConf *conf = &s->conf;
Error *err = NULL;
unsigned i;
if (!conf->conf.blk) {
error_setg(errp, "drive property not set");
return;
}
if (!blk_is_inserted(conf->conf.blk)) {
error_setg(errp, "Device needs media, but drive is empty");
return;
}
if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
conf->num_queues = 1;
}
if (!conf->num_queues) {
error_setg(errp, "num-queues property must be larger than 0");
return;
}
if (conf->queue_size <= 2) {
error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
"must be > 2", conf->queue_size);
return;
}
if (!is_power_of_2(conf->queue_size) ||
conf->queue_size > VIRTQUEUE_MAX_SIZE) {
error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
"must be a power of 2 (max %d)",
conf->queue_size, VIRTQUEUE_MAX_SIZE);
return;
}
if (!blkconf_apply_backend_options(&conf->conf,
!blk_supports_write_perm(conf->conf.blk),
true, errp)) {
return;
}
s->original_wce = blk_enable_write_cache(conf->conf.blk);
if (!blkconf_geometry(&conf->conf, NULL, 65535, 255, 255, errp)) {
return;
}
if (!blkconf_blocksizes(&conf->conf, errp)) {
return;
}
BlockDriverState *bs = blk_bs(conf->conf.blk);
if (bs->bl.zoned != BLK_Z_NONE) {
virtio_add_feature(&s->host_features, VIRTIO_BLK_F_ZONED);
if (bs->bl.zoned == BLK_Z_HM) {
virtio_clear_feature(&s->host_features, VIRTIO_BLK_F_DISCARD);
}
}
if (virtio_has_feature(s->host_features, VIRTIO_BLK_F_DISCARD) &&
(!conf->max_discard_sectors ||
conf->max_discard_sectors > BDRV_REQUEST_MAX_SECTORS)) {
error_setg(errp, "invalid max-discard-sectors property (%" PRIu32 ")"
", must be between 1 and %d",
conf->max_discard_sectors, (int)BDRV_REQUEST_MAX_SECTORS);
return;
}
if (virtio_has_feature(s->host_features, VIRTIO_BLK_F_WRITE_ZEROES) &&
(!conf->max_write_zeroes_sectors ||
conf->max_write_zeroes_sectors > BDRV_REQUEST_MAX_SECTORS)) {
error_setg(errp, "invalid max-write-zeroes-sectors property (%" PRIu32
"), must be between 1 and %d",
conf->max_write_zeroes_sectors,
(int)BDRV_REQUEST_MAX_SECTORS);
return;
}
s->config_size = virtio_get_config_size(&virtio_blk_cfg_size_params,
s->host_features);
virtio_init(vdev, VIRTIO_ID_BLOCK, s->config_size);
s->blk = conf->conf.blk;
s->rq = NULL;
s->sector_mask = (s->conf.conf.logical_block_size / BDRV_SECTOR_SIZE) - 1;
for (i = 0; i < conf->num_queues; i++) {
virtio_add_queue(vdev, conf->queue_size, virtio_blk_handle_output);
}
qemu_coroutine_inc_pool_size(conf->num_queues * conf->queue_size / 2);
virtio_blk_data_plane_create(vdev, conf, &s->dataplane, &err);
if (err != NULL) {
error_propagate(errp, err);
for (i = 0; i < conf->num_queues; i++) {
virtio_del_queue(vdev, i);
}
virtio_cleanup(vdev);
return;
}
/*
* This must be after virtio_init() so virtio_blk_dma_restart_cb() gets
* called after ->start_ioeventfd() has already set blk's AioContext.
*/
s->change =
qdev_add_vm_change_state_handler(dev, virtio_blk_dma_restart_cb, s);
blk_ram_registrar_init(&s->blk_ram_registrar, s->blk);
blk_set_dev_ops(s->blk, &virtio_block_ops, s);
blk_iostatus_enable(s->blk);
add_boot_device_lchs(dev, "/disk@0,0",
conf->conf.lcyls,
conf->conf.lheads,
conf->conf.lsecs);
}在virtio_blk_device_realize函数中，先是通过virtio_init函数初始化VirtIODevice结构。代码片段如下：
virtio_init(vdev, VIRTIO_ID_BLOCK, s->config_size);接下来，根据之前所配置的队列数目conf->num_queues，对于每个队列都调用virtio_add_queue函数来初始化队列。代码片段如下：
for (i = 0; i < conf->num_queues; i++) {
virtio_add_queue(vdev, conf->queue_size, virtio_blk_handle_output);
}先来看一下这个conf->num_queues是在何时、哪里赋值的。
在virtio_blk_device_realize函数中，只有一处相关代码，如下：
conf是从s->conf而来的，而s是通过DeviceState *dev得到的：
VirtIOBlock *s = VIRTIO_BLK(dev);VIRTIO_BLK()定义相关的代码在include/hw/virtio/virtio-blk.h中，如下：
#define TYPE_VIRTIO_BLK "virtio-blk-device"
OBJECT_DECLARE_SIMPLE_TYPE(VirtIOBlock, VIRTIO_BLK)VIRTIO_BLK()的意思和前文书所讲的DEVICE_CLASS()、VIRTIO_DEVICE_CLASS()基本一样，笔者就不在此展开了。直接来看VirtIOBlock结构的定义。
struct VirtIOBlkConf的定义也在include/hw/virtio/virtio-blk.h中，如下：
struct VirtIOBlkConf
{
BlockConf conf;
IOThread *iothread;
char *serial;
uint32_t request_merging;
uint16_t num_queues;
uint16_t queue_size;
bool seg_max_adjust;
bool report_discard_granularity;
uint32_t max_discard_sectors;
uint32_t max_write_zeroes_sectors;
bool x_enable_wce_if_config_wce;
};以笔者目前对于QEMU代码的认知，s->conf.num_queue的值应该是在hw/virtio/virtio-blk-pci.c的virtio_blk_pci_realize函数中赋的，代码如下：
static void virtio_blk_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
{
VirtIOBlkPCI *dev = VIRTIO_BLK_PCI(vpci_dev);
DeviceState *vdev = DEVICE(&dev->vdev);
VirtIOBlkConf *conf = &dev->vdev.conf;
if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
conf->num_queues = virtio_pci_optimal_num_queues(0);
}
if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
vpci_dev->nvectors = conf->num_queues + 1;
}
qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
}
后边随着笔者对于代码认知的深入，会更进一步讲解这一部分。
回到上边的主线：
对于virtio_add_queue函数的解析，放在下一回中。
