QEMU源码全解析 —— virtio（18）

接前一篇文章：

前文书解析了virtio_pci_driver的 probe 回调函数virtio_pci_probe()的前三段代码，本回继续讲解余下代码。为了便于理解和回顾，再次贴出virtio_pci_probe函数源码，在 Linux 内核源码/drivers/ virtio /virtio_pci_common.c中，如下所示：

static int virtio_pci_probe(struct pci_dev *pci_dev,
			    const struct pci_device_id *id)
{
	struct virtio_pci_device *vp_dev, *reg_dev = NULL;
	int rc;
 
	/* allocate our structure and fill it out */
	vp_dev = kzalloc(sizeof(struct virtio_pci_device), GFP_KERNEL);
	if (!vp_dev)
		return -ENOMEM;
 
	pci_set_drvdata(pci_dev, vp_dev);
	vp_dev->vdev.dev.parent = &pci_dev->dev;
	vp_dev->vdev.dev.release = virtio_pci_release_dev;
	vp_dev->pci_dev = pci_dev;
	INIT_LIST_HEAD(&vp_dev->virtqueues);
	spin_lock_init(&vp_dev->lock);
 
	/* enable the device */
	rc = pci_enable_device(pci_dev);
	if (rc)
		goto err_enable_device;
 
	if (force_legacy) {
		rc = virtio_pci_legacy_probe(vp_dev);
		/* Also try modern mode if we can't map BAR0 (no IO space). */
		if (rc == -ENODEV || rc == -ENOMEM)
			rc = virtio_pci_modern_probe(vp_dev);
		if (rc)
			goto err_probe;
	} else {
		rc = virtio_pci_modern_probe(vp_dev);
		if (rc == -ENODEV)
			rc = virtio_pci_legacy_probe(vp_dev);
		if (rc)
			goto err_probe;
	}
 
	pci_set_master(pci_dev);
 
	vp_dev->is_legacy = vp_dev->ldev.ioaddr ? true : false;
 
	rc = register_virtio_device(&vp_dev->vdev);
	reg_dev = vp_dev;
	if (rc)
		goto err_register;
 
	return 0;
 
err_register:
	if (vp_dev->is_legacy)
		virtio_pci_legacy_remove(vp_dev);
	else
		virtio_pci_modern_remove(vp_dev);
err_probe:
	pci_disable_device(pci_dev);
err_enable_device:
	if (reg_dev)
		put_device(&vp_dev->vdev.dev);
	else
		kfree(vp_dev);
	return rc;
}

（4）调用pci_enable_device函数使能该 PCI 设备。代码片段如下：

    /* enable the device */
	rc = pci_enable_device(pci_dev);
	if (rc)
		goto err_enable_device;

pci_dev是virtio_pci_probe函数的第1个参数，类型为struct pci_dev。

pci_enable_device函数在Linux内核源码/drivers/pci/pci.c中，代码如下：

/**
 * pci_enable_device - Initialize device before it's used by a driver.
 * @dev: PCI device to be initialized
 *
 * Initialize device before it's used by a driver. Ask low-level code
 * to enable I/O and memory. Wake up the device if it was suspended.
 * Beware, this function can fail.
 *
 * Note we don't actually enable the device many times if we call
 * this function repeatedly (we just increment the count).
 */
int pci_enable_device(struct pci_dev *dev)
{
	return pci_enable_device_flags(dev, IORESOURCE_MEM | IORESOURCE_IO);
}
EXPORT_SYMBOL(pci_enable_device);

pci_enable_device_flags函数在同文件中，代码如下：

static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
{
	struct pci_dev *bridge;
	int err;
	int i, bars = 0;
 
	/*
	 * Power state could be unknown at this point, either due to a fresh
	 * boot or a device removal call.  So get the current power state
	 * so that things like MSI message writing will behave as expected
	 * (e.g. if the device really is in D0 at enable time).
	 */
	pci_update_current_state(dev, dev->current_state);
 
	if (atomic_inc_return(&dev->enable_cnt) > 1)
		return 0;		/* already enabled */
 
	bridge = pci_upstream_bridge(dev);
	if (bridge)
		pci_enable_bridge(bridge);
 
	/* only skip sriov related */
	for (i = 0; i <= PCI_ROM_RESOURCE; i++)
		if (dev->resource[i].flags & flags)
			bars |= (1 << i);
	for (i = PCI_BRIDGE_RESOURCES; i < DEVICE_COUNT_RESOURCE; i++)
		if (dev->resource[i].flags & flags)
			bars |= (1 << i);
 
	err = do_pci_enable_device(dev, bars);
	if (err < 0)
		atomic_dec(&dev->enable_cnt);
	return err;
}

（5）接下来，调用virtio_pci_legacy_probe或者virtio_pci_modern_probe函数来初始化该PCI设备对应的virtio设备。代码片段如下：

    if (force_legacy) {
		rc = virtio_pci_legacy_probe(vp_dev);
		/* Also try modern mode if we can't map BAR0 (no IO space). */
		if (rc == -ENODEV || rc == -ENOMEM)
			rc = virtio_pci_modern_probe(vp_dev);
		if (rc)
			goto err_probe;
	} else {
		rc = virtio_pci_modern_probe(vp_dev);
		if (rc == -ENODEV)
			rc = virtio_pci_legacy_probe(vp_dev);
		if (rc)
			goto err_probe;
	}

如果force_legacy标志为true，则先调用virtio_pci_legacy_probe函数，出现错误（-ENODEV或-ENOMEM）时再调用virtio_pci_modern_probe函数；如果force_legacy标志为false，则正好反过来，先调用virtio_pci_modern_probe函数，出现错误（-ENODEV或-ENOMEM）时再调用virtio_pci_legacy_probe函数。

force_legacy是模块参数，也在Linux内核源码/drivers/virtio/virtio_pci_common.c中，相关代码如下：

static bool force_legacy = false;
 
#if IS_ENABLED(CONFIG_VIRTIO_PCI_LEGACY)
module_param(force_legacy, bool, 0444);
MODULE_PARM_DESC(force_legacy,
		 "Force legacy mode for transitional virtio 1 devices");
#endif

• virtio_pci_legacy_probe

virtio_pci_legacy_probe函数在Linux内核源码/drivers/virtio/virtio_pci_legacy.c中，代码如下：

/* the PCI probing function */
int virtio_pci_legacy_probe(struct virtio_pci_device *vp_dev)
{
	struct virtio_pci_legacy_device *ldev = &vp_dev->ldev;
	struct pci_dev *pci_dev = vp_dev->pci_dev;
	int rc;
 
	ldev->pci_dev = pci_dev;
 
	rc = vp_legacy_probe(ldev);
	if (rc)
		return rc;
 
	vp_dev->isr = ldev->isr;
	vp_dev->vdev.id = ldev->id;
 
	vp_dev->vdev.config = &virtio_pci_config_ops;
 
	vp_dev->config_vector = vp_config_vector;
	vp_dev->setup_vq = setup_vq;
	vp_dev->del_vq = del_vq;
 
	return 0;
}

• virtio_pci_modern_probe

virtio_pci_modern_probe函数在Linux内核源码/drivers/virtio/virtio_pci_modern.c中，代码如下：

/* the PCI probing function */
int virtio_pci_modern_probe(struct virtio_pci_device *vp_dev)
{
	struct virtio_pci_modern_device *mdev = &vp_dev->mdev;
	struct pci_dev *pci_dev = vp_dev->pci_dev;
	int err;
 
	mdev->pci_dev = pci_dev;
 
	err = vp_modern_probe(mdev);
	if (err)
		return err;
 
	if (mdev->device)
		vp_dev->vdev.config = &virtio_pci_config_ops;
	else
		vp_dev->vdev.config = &virtio_pci_config_nodev_ops;
 
	vp_dev->config_vector = vp_config_vector;
	vp_dev->setup_vq = setup_vq;
	vp_dev->del_vq = del_vq;
	vp_dev->isr = mdev->isr;
	vp_dev->vdev.id = mdev->id;
 
	return 0;
}

下一回将对这两个函数尤其是virtio_pci_modern_probe()进行详细解析。