On Sat, Jan 5, 2019 at 10:42 AM Peter Maydell <peter.mayd...@linaro.org> wrote: > > 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.
Thanks for the insight, Peter. That make sense!! Thanks, Jintack > > > 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 >
