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sameo/virtio-probe.md

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virtio-probe.md

virtio is a kernel virtual bus. From drivers/virtio/virtio.c:

static struct bus_type virtio_bus = {
	.name  = "virtio",
	.match = virtio_dev_match,
	.dev_groups = virtio_dev_groups,
	.uevent = virtio_uevent,
	.probe = virtio_dev_probe,
	.remove = virtio_dev_remove,
};

PCI

From drivers/virtio/virtio_pci_common.c:

/* Qumranet donated their vendor ID for devices 0x1000 thru 0x10FF. */
static const struct pci_device_id virtio_pci_id_table[] = {
	{ PCI_DEVICE(PCI_VENDOR_ID_REDHAT_QUMRANET, PCI_ANY_ID) },
	{ 0 }
};

static int virtio_pci_probe(struct pci_dev *pci_dev,
			    const struct pci_device_id *id)
{
	struct virtio_pci_device *vp_dev;
	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);

	rc = register_virtio_device(&vp_dev->vdev);

register_virtio_device() registers a virtio specific device on the virtio bus. The virtio device struct:

struct virtio_device {
	int index;
	bool failed;
	bool config_enabled;
	bool config_change_pending;
	spinlock_t config_lock;
	struct device dev;
	struct virtio_device_id id;
	const struct virtio_config_ops *config;
	const struct vringh_config_ops *vringh_config;
	struct list_head vqs;
	u64 features;
	void *priv;
};

And virtio_device_id is the ID used for the driver <-> device match

For example, virtio net is at drivers/net/virtio_net.c and identifies itself as:

static struct virtio_device_id id_table[] = {
	{ VIRTIO_ID_NET, VIRTIO_DEV_ANY_ID },
	{ 0 },
};

All virtio IDs are defined in the userspace API at include/uapi/linux/virtio_ids.h:

#define VIRTIO_ID_NET		1 /* virtio net */
#define VIRTIO_ID_BLOCK		2 /* virtio block */
#define VIRTIO_ID_CONSOLE	3 /* virtio console */
#define VIRTIO_ID_RNG		4 /* virtio rng */
#define VIRTIO_ID_BALLOON	5 /* virtio balloon */
#define VIRTIO_ID_RPMSG		7 /* virtio remote processor messaging */
#define VIRTIO_ID_SCSI		8 /* virtio scsi */
#define VIRTIO_ID_9P		9 /* 9p virtio console */
#define VIRTIO_ID_RPROC_SERIAL 11 /* virtio remoteproc serial link */
#define VIRTIO_ID_CAIF	       12 /* Virtio caif */
#define VIRTIO_ID_GPU          16 /* virtio GPU */
#define VIRTIO_ID_INPUT        18 /* virtio input */
#define VIRTIO_ID_VSOCK        19 /* virtio vsock transport */
#define VIRTIO_ID_CRYPTO       20 /* virtio crypto */

MMIO

virtio MMIO provides the device configuration through memory mapped registers:

From drivers/virtio/virtio_mmio.c:

	/* Check magic value */
	magic = readl(vm_dev->base + VIRTIO_MMIO_MAGIC_VALUE);
	if (magic != ('v' | 'i' << 8 | 'r' << 16 | 't' << 24)) {
		dev_warn(&pdev->dev, "Wrong magic value 0x%08lx!\n", magic);
		return -ENODEV;
	}

	/* Check device version */
	vm_dev->version = readl(vm_dev->base + VIRTIO_MMIO_VERSION);
	if (vm_dev->version < 1 || vm_dev->version > 2) {
		dev_err(&pdev->dev, "Version %ld not supported!\n",
				vm_dev->version);
		return -ENXIO;
	}

	vm_dev->vdev.id.device = readl(vm_dev->base + VIRTIO_MMIO_DEVICE_ID);
	if (vm_dev->vdev.id.device == 0) {
		/*
		 * virtio-mmio device with an ID 0 is a (dummy) placeholder
		 * with no function. End probing now with no error reported.
		 */
		return -ENODEV;
	}
	vm_dev->vdev.id.vendor = readl(vm_dev->base + VIRTIO_MMIO_VENDOR_ID);

And then register the corresponding device on the virtio bus:

	return register_virtio_device(&vm_dev->vdev);

The MMIO virtio device registers offsets are defined at include/uapi/linux/virtio_mmio.h:

/*
 * Control registers
 */

/* Magic value ("virt" string) - Read Only */
#define VIRTIO_MMIO_MAGIC_VALUE		0x000

/* Virtio device version - Read Only */
#define VIRTIO_MMIO_VERSION		0x004

/* Virtio device ID - Read Only */
#define VIRTIO_MMIO_DEVICE_ID		0x008

/* Virtio vendor ID - Read Only */
#define VIRTIO_MMIO_VENDOR_ID		0x00c

/* Bitmask of the features supported by the device (host)
 * (32 bits per set) - Read Only */
#define VIRTIO_MMIO_DEVICE_FEATURES	0x010

/* Device (host) features set selector - Write Only */
#define VIRTIO_MMIO_DEVICE_FEATURES_SEL	0x014

/* Bitmask of features activated by the driver (guest)
 * (32 bits per set) - Write Only */
#define VIRTIO_MMIO_DRIVER_FEATURES	0x020

/* Activated features set selector - Write Only */
#define VIRTIO_MMIO_DRIVER_FEATURES_SEL	0x024


#ifndef VIRTIO_MMIO_NO_LEGACY /* LEGACY DEVICES ONLY! */

/* Guest's memory page size in bytes - Write Only */
#define VIRTIO_MMIO_GUEST_PAGE_SIZE	0x028

#endif


/* Queue selector - Write Only */
#define VIRTIO_MMIO_QUEUE_SEL		0x030

/* Maximum size of the currently selected queue - Read Only */
#define VIRTIO_MMIO_QUEUE_NUM_MAX	0x034

/* Queue size for the currently selected queue - Write Only */
#define VIRTIO_MMIO_QUEUE_NUM		0x038


#ifndef VIRTIO_MMIO_NO_LEGACY /* LEGACY DEVICES ONLY! */

/* Used Ring alignment for the currently selected queue - Write Only */
#define VIRTIO_MMIO_QUEUE_ALIGN		0x03c

/* Guest's PFN for the currently selected queue - Read Write */
#define VIRTIO_MMIO_QUEUE_PFN		0x040

#endif


/* Ready bit for the currently selected queue - Read Write */
#define VIRTIO_MMIO_QUEUE_READY		0x044

/* Queue notifier - Write Only */
#define VIRTIO_MMIO_QUEUE_NOTIFY	0x050

/* Interrupt status - Read Only */
#define VIRTIO_MMIO_INTERRUPT_STATUS	0x060

/* Interrupt acknowledge - Write Only */
#define VIRTIO_MMIO_INTERRUPT_ACK	0x064

/* Device status register - Read Write */
#define VIRTIO_MMIO_STATUS		0x070

/* Selected queue's Descriptor Table address, 64 bits in two halves */
#define VIRTIO_MMIO_QUEUE_DESC_LOW	0x080
#define VIRTIO_MMIO_QUEUE_DESC_HIGH	0x084

/* Selected queue's Available Ring address, 64 bits in two halves */
#define VIRTIO_MMIO_QUEUE_AVAIL_LOW	0x090
#define VIRTIO_MMIO_QUEUE_AVAIL_HIGH	0x094

/* Selected queue's Used Ring address, 64 bits in two halves */
#define VIRTIO_MMIO_QUEUE_USED_LOW	0x0a0
#define VIRTIO_MMIO_QUEUE_USED_HIGH	0x0a4

/* Configuration atomicity value */
#define VIRTIO_MMIO_CONFIG_GENERATION	0x0fc

/* The config space is defined by each driver as
 * the per-driver configuration space - Read Write */
#define VIRTIO_MMIO_CONFIG		0x100



/*
 * Interrupt flags (re: interrupt status & acknowledge registers)
 */

#define VIRTIO_MMIO_INT_VRING		(1 << 0)
#define VIRTIO_MMIO_INT_CONFIG		(1 << 1)
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