On Fri, 4 Jan 2019 at 20:23, Jintack Lim <jint...@cs.columbia.edu> wrote: > I was wondering why one virtio-pci device has two different > DeviceState? - one directly from VirtIOPCIProxy and the other from > VirtIO<dev type> such as VirtIONet. As an example, they are denoted as > qdev and vdev respectively in virtio_net_pci_realize().
It's been a while since I looked at this, but there are two basic issues underlying the weird way virtio devices are set up: (1) PCI is not the only "transport" -- the VirtIONet etc are shared with other transports like MMIO or the S390 ones (2) retaining back-compatibility matters a lot here: we need command lines to still work, and also the migration data stream needs to stay compatible Some of the way the devices are reflects the way we started with a design where there was only a single device (eg the pci virtio-net device) and then refactored it to support multiple transports while retaining back compatibility. > I thought that just one DeviceState is enough for any device in QEMU. > Maybe I'm missing something fundamental here. This isn't generally true, it's just that a lot of our devices are of the simple straightforward kind where that's true. It's also possible for an implementation of a device to be as a combination of other devices, which is what we have here. virtio-pci-net is-a PCIDevice (which in turn is-a Device), but it has-a VirtIONet device (which is-a Device) as part of its implementation. (It's also possible to manually create the pci transport and the virtio-net backend separately and connect them together without the virtio-pci-net device at all. That's more often used with non-pci transports but it works for pci too.) You can also see a similar thing with a lot of the "container" SoC objects like TYPE_ASPEED_SOC, which is a subclass of DeviceState, but is implemented using a dozen different objects all of which are themselves DeviceState subclasses. thanks -- PMM