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

[Michael S. Tsirkin]

On Mon, Mar 22, 2010 at 09:49:03AM -0500, Anthony Liguori wrote:
> 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.

Does this boil down to qdev wanting to be the 1st field
in the structure, again? We can just lift that limitation.

>> 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.

No. OTOH whether we want a shared vector or a per-vq vector
might depend on the device being used.

> Regards,
>
> Anthony Liguori
>
>> Paul
>>