Re: [Qemu-devel] Virtio refactoring.

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Tue, 13 Nov 2012 15:27:57 +0100
KONRAD Frédéric <fred.konrad@greensocs.com> wrote:

> To fix this, an idea is to use a new qbus named VirtioBus to link virtio-pci
> or virtio-mmio with all the virtio backend ( VirtioDevice ). So 
> "virtio-pci" and
> "virtio-mmio" will have a VirtioBus.

Just to spell this out:

We'd go from

system bus
-> virtio transport bridge dev (virtio-xxx-bridge)
   -> virtio transport bus (virtio-xxx-bus)
      -> virtio transport dev (virtio-<type>-xxx)

to

system bus
-> virtio transport bridge dev (virtio-bridge-xxx)
   -> virtio bus (virtio-bus-xxx)
      -> virtio dev (virtio-<type>-xxx)

?

Would this also mean we could have several virtio-busses with different
transports?

> 
> To do that we will do the following things in the right order :
>      * Introduce a new VirtioBus ( same way as scsi-bus.c ), with 
> VirtIODevice
>        interface :
>           -> callback to completely abstract the VirtioDevice from 
> VirtioPCI.
>           -> for the queue, load/save the queue/config, features, ..., 
> other ?
>      * Add a VirtioBus to the VirtioPCIProxy. ( virtio-pci.c ) :
>           -> moving all to the newer callback.
>      * For each of the virtio-device : ( virtio-x.c )
>           -> making a separate class for virtio-x which is a VirtioDevice.
>           -> making a virtio-x-pci which has a virtio-x.
>      * Create virtio-mmio ( virtio-mmio.c ).
> 
> Is it the right approach ? Do I miss something ?

What of the alias handling? Can this be killed once everything has been
converted?

> 
> When it will work, we must be sure of :
> 
> -> migration compatibility.
> -> not breaking the s390 transport.
> -> compatibility with s390 ccw.

There shouldn't be major problems rebasing the virtio-ccw code on top
of this rework (though I'd probably try to keep the basic channel I/O
support separate from this patchset).