[RFC] Seeking advice on support for generic vDPA device

[jiangdongxu <jiangdongxu1@xxxxxxxxxx>]

vDPA devices allow high-performance devices in a virtual machine by
providing a wire-speed data path. These devices require a vendor-specific host
driver but the data path follows the virtio specification.

The support for generic-vdpa device was recently added to qemu in v8.0.0-rc version.
https://gitlab.com/qemu-project/qemu/-/commit/b430a2bd2303b9940cc80ec746887f463a25fc5c

This allows libvirt to support these deviecs regardless of device type. 
Currently, libvirt supports vDPA network devices, We need a new solution
to support generic-vdpa devices.

At present, we have considered several options.

Solution A:
    Add type vdpa under the hostdev device，the device xml looks like
    <devices>
        <hostdev mode='subsystem' type='vdpa'>
            <source dev='/dev/vhost-vdpa-0'/>
        </hostdev>
    </devices>
    This solution is recommended. This configuration requires only that the
    device supports the vDPA framework, easy for future expansion and same 
    as the qemu solution design idea.

Solution B:
    The current libvirt mode is used to add vDPA devices under different devices.
    device xml looks like:
    network:
    <devices>
      <interface type='generic-vdpa'>
        <source dev='/dev/vhost-vdpa-0'/>
      </interface>
    </devices>

    storage:
    <devices>
        <disk type='generic-vdpa’>
          <source dev='/dev/vhost-vdpa-0'/>
        </disk>
    </devices>

    The network interface has the vdpa type. Therefore, a new type is required to
    distinguish the "generic-vdpa" type. However, this method differentiates
    device types, which is not convenient for the expansion of other types
    of vDPA devices. Not recommended.

Solution C:
    Referring to PCI passthrough, Add the driver type (appropriate vendor-specific driver) to the driver of the PCI device.
    The unbind/bind of the vDPA device driver and the creation of VDPA device are implemented by libvirt.
    device xml looks like:
    <devices>
        <hostdev mode='subsystem' type='pci' managed='yes'>
            <driver name='vp-vdpa'/>
            <source>
                <address domain='0x0000' bus='0x04' slot='0x01' function='0x01'/>
            </source>
        </hostdev>
    </devices>
    This solution is related to the vendor-specific driver and supports only the vdpa created by the PCI device. 
    vdpa devices created in vduse mode are not supported. This solution is not universal and is not recommended.

    
What's your proposal?