接前一篇文章：

上回书继续讲解virtio_pci_driver的probe回调函数virtio_pci_probe()，在讲到第5段代码的时候，

    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;}

引出来两个函数：virtio_pci_legacy_probe和virtio_pci_modern_probe。本回就来对它们进行解析。当然，由于legacy已成过去，因此重点围绕virtio_pci_modern_probe函数进行解析，捎带手地也讲一下virtio_pci_legacy_probe()。为了便于理解，再次贴出两个函数的源码：

• 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;elsevp_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;
}

virtio_pci_modern_probe函数中最主要的是调用了vp_modern_probe函数，其在Linux内核源码/drivers/virtio/virtio_pci_modern_dev.c中，代码如下：

/** vp_modern_probe: probe the modern virtio pci device, note that the* caller is required to enable PCI device before calling this function.* @mdev: the modern virtio-pci device** Return 0 on succeed otherwise fail*/
int vp_modern_probe(struct virtio_pci_modern_device *mdev)
{struct pci_dev *pci_dev = mdev->pci_dev;int err, common, isr, notify, device;u32 notify_length;u32 notify_offset;check_offsets();/* We only own devices >= 0x1000 and <= 0x107f: leave the rest. */if (pci_dev->device < 0x1000 || pci_dev->device > 0x107f)return -ENODEV;if (pci_dev->device < 0x1040) {/* Transitional devices: use the PCI subsystem device id as* virtio device id, same as legacy driver always did.*/mdev->id.device = pci_dev->subsystem_device;} else {/* Modern devices: simply use PCI device id, but start from 0x1040. */mdev->id.device = pci_dev->device - 0x1040;}mdev->id.vendor = pci_dev->subsystem_vendor;/* check for a common config: if not, use legacy mode (bar 0). */common = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_COMMON_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);if (!common) {dev_info(&pci_dev->dev,"virtio_pci: leaving for legacy driver\n");return -ENODEV;}/* If common is there, these should be too... */isr = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_ISR_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);notify = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_NOTIFY_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);if (!isr || !notify) {dev_err(&pci_dev->dev,"virtio_pci: missing capabilities %i/%i/%i\n",common, isr, notify);return -EINVAL;}err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));if (err)err = dma_set_mask_and_coherent(&pci_dev->dev,DMA_BIT_MASK(32));if (err)dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");/* Device capability is only mandatory for devices that have* device-specific configuration.*/device = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_DEVICE_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);err = pci_request_selected_regions(pci_dev, mdev->modern_bars,"virtio-pci-modern");if (err)return err;err = -EINVAL;mdev->common = vp_modern_map_capability(mdev, common,sizeof(struct virtio_pci_common_cfg), 4,0, sizeof(struct virtio_pci_common_cfg),NULL, NULL);if (!mdev->common)goto err_map_common;mdev->isr = vp_modern_map_capability(mdev, isr, sizeof(u8), 1,0, 1,NULL, NULL);if (!mdev->isr)goto err_map_isr;/* Read notify_off_multiplier from config space. */pci_read_config_dword(pci_dev,notify + offsetof(struct virtio_pci_notify_cap,notify_off_multiplier),&mdev->notify_offset_multiplier);/* Read notify length and offset from config space. */pci_read_config_dword(pci_dev,notify + offsetof(struct virtio_pci_notify_cap,cap.length),&notify_length);pci_read_config_dword(pci_dev,notify + offsetof(struct virtio_pci_notify_cap,cap.offset),&notify_offset);/* We don't know how many VQs we'll map, ahead of the time.* If notify length is small, map it all now.* Otherwise, map each VQ individually later.*/if ((u64)notify_length + (notify_offset % PAGE_SIZE) <= PAGE_SIZE) {mdev->notify_base = vp_modern_map_capability(mdev, notify,2, 2,0, notify_length,&mdev->notify_len,&mdev->notify_pa);if (!mdev->notify_base)goto err_map_notify;} else {mdev->notify_map_cap = notify;}/* Again, we don't know how much we should map, but PAGE_SIZE* is more than enough for all existing devices.*/if (device) {mdev->device = vp_modern_map_capability(mdev, device, 0, 4,0, PAGE_SIZE,&mdev->device_len,NULL);if (!mdev->device)goto err_map_device;}return 0;err_map_device:if (mdev->notify_base)pci_iounmap(pci_dev, mdev->notify_base);
err_map_notify:pci_iounmap(pci_dev, mdev->isr);
err_map_isr:pci_iounmap(pci_dev, mdev->common);
err_map_common:pci_release_selected_regions(pci_dev, mdev->modern_bars);return err;
}
EXPORT_SYMBOL_GPL(vp_modern_probe);

实际上在老版本KVM即Linux内核代码中，vp_modern_probe函数中的内容绝大多数是直接放在virtio_pci_modern_probe函数中的，后来才单独封了这样一个函数。

（1）vp_modern_probe首先设置了virtio设备的verdor ID和device ID。代码片段如下：

    /* We only own devices >= 0x1000 and <= 0x107f: leave the rest. */if (pci_dev->device < 0x1000 || pci_dev->device > 0x107f)return -ENODEV;if (pci_dev->device < 0x1040) {/* Transitional devices: use the PCI subsystem device id as* virtio device id, same as legacy driver always did.*/mdev->id.device = pci_dev->subsystem_device;} else {/* Modern devices: simply use PCI device id, but start from 0x1040. */mdev->id.device = pci_dev->device - 0x1040;}mdev->id.vendor = pci_dev->subsystem_vendor;

值得注意的是，virtio PCI代理设备的device iD就是前文书（参见QEMU源码全解析 —— virtio（14））在讲virtio_pci_device_plugged函数（QEMU源码中）时设置的PCI_DEVICE_ID_VIRTIO_10_BASE+VIRTIO_ID_BALLOON，即0x1040+5。

所以，这里virtio设备的device ID（mdev->id.device）就是0x1040+5-0x1040=5，也就代表了VIRTIO_ID_BALLOON。

（2）接下来，调用多次virtio_pci_find_capability函数来发现virtio PCI代理设备的pci capability。代码片段如下：

/* check for a common config: if not, use legacy mode (bar 0). */common = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_COMMON_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);if (!common) {dev_info(&pci_dev->dev,"virtio_pci: leaving for legacy driver\n");return -ENODEV;}/* If common is there, these should be too... */isr = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_ISR_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);notify = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_NOTIFY_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);if (!isr || !notify) {dev_err(&pci_dev->dev,"virtio_pci: missing capabilities %i/%i/%i\n",common, isr, notify);return -EINVAL;}err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));if (err)err = dma_set_mask_and_coherent(&pci_dev->dev,DMA_BIT_MASK(32));if (err)dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");/* Device capability is only mandatory for devices that have* device-specific configuration.*/device = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_DEVICE_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);

这也是在（QEMU源码）virtio_pci_device_plugged函数中写入到virtio PCI代理设备的配置空间中的，参见QEMU源码全解析 —— virtio（14）和QEMU源码全解析 —— virtio（15）。

（3）virtio_pci_find_capability函数找到所属的PCI BAR，然后写入到virt_pci_device的modern_bars成员中。代码片段如下：

    /* Device capability is only mandatory for devices that have* device-specific configuration.*/device = virtio_pci_find_capability(pci_dev, VIRTIO_PCI_CAP_DEVICE_CFG,IORESOURCE_IO | IORESOURCE_MEM,&mdev->modern_bars);

从（QEMU源码）virtio_pci_realize函数中可以知道这个modern_bars是1<<4，如下图所示：

（4）接着，pci_request_selected_regions函数就将virtio PCI代理设备的BAR地址空间保留出来了。代码片段如下：

    err = pci_request_selected_regions(pci_dev, mdev->modern_bars,"virtio-pci-modern");if (err)return err;

（5）