Re: [Qemu-devel] Re: [PATCH 0/9] Virtio cleanups

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/22/2010 08:30 AM, Paul Brook wrote:
>> A VirtIOBlock device cannot be a VirtIODevice while being a
>> VirtIOPCIProxy (proxy is a poor name, btw).
>>
>> It really ought to be:
>>
>> DeviceState ->  VirtIODevice ->  VirtIOBlock
>>
>> and:
>>
>> PCIDevice ->  VirtIOPCI : implements VirtIOBus
>>
>> The interface between the VirtIODevice and VirtIOBus is the virtio
>> transport.
>>
>> The main reason a separate bus is needed is the same reason it's needed
>> in Linux.  VirtIOBlock has to be tied to some bus.  It cannot be tied to
>> the PCI bus without having it be part of the implementation detail.
>> Introducing another bus type fixes this (and it's what we do in the
>>   kernel).
>>      
> Why does virtio need a device state and bus at all?
>    

Because you need VirtIOBlock to have qdev properties that can be set.

You also need VirtIOPCI to have separate qdev properties that can be set.

> Can't it just be an internal implementation interface, which happens to be
> used by all devices that happen to exposed a block device over a virtio
> transport?
>    

Theoretically, yes, but given the rest of the infrastructure's 
interaction with qdev, making it a device makes the most sense IMHO.

> If you have a virtio bus then IMO the PCI bridge device should be independent
> of the virtio device that is connected to it.
>    

Yes, that's the point I'm making.  IOW, there shouldn't be a 
"virtio-net-pci" device.  Instead, there should be a "virtio-pci" device 
that implements a VirtIOBus and then we add a single VirtIODevice to it 
like "virtio-net".

For something like MSI vector support, virtio-net really should have no 
knowledge of MSI-x.  Instead, you should specific nvectors to virtio-pci 
and then virtio-pci should decide how to tie individual queue 
notifications to the amount of MSI vectors it has.

I can't envision any reason why we would ever want to have two MSI 
vectors for a given queue.

Regards,

Anthony Liguori

> Paul
>