# Writing VirtIO backends for QEMU

This document attempts to outline the information a developer needs to know to write device emulations in QEMU. It is specifically focused on implementing VirtIO devices. For VirtIO the frontend is the driver running on the guest. The backend is the everything that QEMU needs to do to handle the emulation of the VirtIO device. This can be done entirely in QEMU, divided between QEMU and the kernel (vhost) or handled by a separate process which is configured by QEMU (vhost-user).

## VirtIO Transports

VirtIO supports a number of different transports. While the details of the configuration and operation of the device will generally be the same QEMU represents them as different devices depending on the transport they use. For example -device virtio-foo represents the foo device using mmio and -device virtio-foo-pci is the same class of device using the PCI transport.

## Using the QEMU Object Model (QOM)

Generally all devices in QEMU are super classes of `TYPE_DEVICE` however VirtIO devices should be based on `TYPE_VIRTIO_DEVICE` which itself is derived from the base class. For example:

    static const TypeInfo virtio_blk_info = {
        .name = TYPE_VIRTIO_BLK,
        .parent = TYPE_VIRTIO_DEVICE,
        .instance_size = sizeof(VirtIOBlock),
        .instance_init = virtio_blk_instance_init,
        .class_init = virtio_blk_class_init,
    };

The author may decide to have a more expansive class hierarchy to support multiple device types. For example the Virtio GPU device:

    static const TypeInfo virtio_gpu_base_info = {
        .name = TYPE_VIRTIO_GPU_BASE,
        .parent = TYPE_VIRTIO_DEVICE,
        .instance_size = sizeof(VirtIOGPUBase),
        .class_size = sizeof(VirtIOGPUBaseClass),
        .class_init = virtio_gpu_base_class_init,
        .abstract = true
    };

    static const TypeInfo vhost_user_gpu_info = {
        .name = TYPE_VHOST_USER_GPU,
        .parent = TYPE_VIRTIO_GPU_BASE,
        .instance_size = sizeof(VhostUserGPU),
        .instance_init = vhost_user_gpu_instance_init,
        .instance_finalize = vhost_user_gpu_instance_finalize,
        .class_init = vhost_user_gpu_class_init,
    };

    static const TypeInfo virtio_gpu_info = {
        .name = TYPE_VIRTIO_GPU,
        .parent = TYPE_VIRTIO_GPU_BASE,
        .instance_size = sizeof(VirtIOGPU),
        .class_size = sizeof(VirtIOGPUClass),
        .class_init = virtio_gpu_class_init,
    };

defines a base class for the VirtIO GPU and then specialises two versions, one for the internal implementation and the other for the vhost-user version.

### VirtIOPCIProxy

[AJB: the following is supposition and welcomes more informed opinions]

Probably due to legacy from the pre-QOM days PCI VirtIO devices don’t follow the normal hierarchy. Instead the a standalone object is based on the VirtIOPCIProxy class and the specific VirtIO instance is manually instantiated:

    /*
     * virtio-blk-pci: This extends VirtioPCIProxy.
     */
    #define TYPE_VIRTIO_BLK_PCI "virtio-blk-pci-base"
    DECLARE_INSTANCE_CHECKER(VirtIOBlkPCI, VIRTIO_BLK_PCI,
                             TYPE_VIRTIO_BLK_PCI)

    struct VirtIOBlkPCI {
        VirtIOPCIProxy parent_obj;
        VirtIOBlock vdev;
    };

    static const Property virtio_blk_pci_properties[] = {
        DEFINE_PROP_UINT32("class", VirtIOPCIProxy, class_code, 0),
        DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags,
                        VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, true),
        DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors,
                           DEV_NVECTORS_UNSPECIFIED),
    };

    static void virtio_blk_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
    {
        VirtIOBlkPCI *dev = VIRTIO_BLK_PCI(vpci_dev);
        DeviceState *vdev = DEVICE(&dev->vdev);

        ...

        qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
    }

    static void virtio_blk_pci_class_init(ObjectClass *klass, const void *data)
    {
        DeviceClass *dc = DEVICE_CLASS(klass);
        VirtioPCIClass *k = VIRTIO_PCI_CLASS(klass);
        PCIDeviceClass *pcidev_k = PCI_DEVICE_CLASS(klass);

        set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
        device_class_set_props(dc, virtio_blk_pci_properties);
        k->realize = virtio_blk_pci_realize;
        pcidev_k->vendor_id = PCI_VENDOR_ID_REDHAT_QUMRANET;
        pcidev_k->device_id = PCI_DEVICE_ID_VIRTIO_BLOCK;
        pcidev_k->revision = VIRTIO_PCI_ABI_VERSION;
        pcidev_k->class_id = PCI_CLASS_STORAGE_SCSI;
    }

    static void virtio_blk_pci_instance_init(Object *obj)
    {
        VirtIOBlkPCI *dev = VIRTIO_BLK_PCI(obj);

        virtio_instance_init_common(obj, &dev->vdev, sizeof(dev->vdev),
                                    TYPE_VIRTIO_BLK);
        object_property_add_alias(obj, "bootindex", OBJECT(&dev->vdev),
                                  "bootindex");
    }

    static const VirtioPCIDeviceTypeInfo virtio_blk_pci_info = {
        .base_name              = TYPE_VIRTIO_BLK_PCI,
        .generic_name           = "virtio-blk-pci",
        .transitional_name      = "virtio-blk-pci-transitional",
        .non_transitional_name  = "virtio-blk-pci-non-transitional",
        .instance_size = sizeof(VirtIOBlkPCI),
        .instance_init = virtio_blk_pci_instance_init,
        .class_init    = virtio_blk_pci_class_init,
    };

Here you can see the instance_init has to manually instantiate the underlying `TYPE_VIRTIO_BLOCK` object and link an alias for one of it’s properties to the PCI device.

## Back End Implementations

There are a number of places where the implementation of the backend can be done:

- in QEMU itself

- in the host kernel (a.k.a vhost)

- in a separate process (a.k.a. vhost-user)

### vhost_ops vs TYPE_VHOST_USER_BACKEND

There are two choices to how to implement vhost code. Most of the code which has to work with either vhost or vhost-user uses `vhost_dev_init()` to instantiate the appropriate backend. This means including a `struct vhost_dev` in the main object structure.

For vhost-user devices you also need to add code to track the initialisation of the `chardev` device used for the control socket between QEMU and the external vhost-user process.

If you only need to implement a vhost-user backed the other option is a use a QOM-ified version of vhost-user.

    static void
    vhost_user_gpu_instance_init(Object *obj)
    {
        VhostUserGPU *g = VHOST_USER_GPU(obj);

        g->vhost = VHOST_USER_BACKEND(object_new(TYPE_VHOST_USER_BACKEND));
        object_property_add_alias(obj, "chardev",
                                  OBJECT(g->vhost), "chardev");
    }

    static const TypeInfo vhost_user_gpu_info = {
        .name = TYPE_VHOST_USER_GPU,
        .parent = TYPE_VIRTIO_GPU_BASE,
        .instance_size = sizeof(VhostUserGPU),
        .instance_init = vhost_user_gpu_instance_init,
        .instance_finalize = vhost_user_gpu_instance_finalize,
        .class_init = vhost_user_gpu_class_init,
    };

Using it this way entails adding a `struct VhostUserBackend` to your core object structure and manually instantiating the backend. This sub-structure tracks both the `vhost_dev` and `CharDev` types needed for the connection. Instead of calling `vhost_dev_init` you would call `vhost_user_backend_dev_init` which does what is needed on your behalf.