Re: [Qemu-devel] [PATCH 0/8] Add virtio-mmio and use it in vexpress

[Alexander Graf]

On 08.07.2013, at 15:23, Peter Maydell wrote:

> On 8 July 2013 14:16, Alexander Graf <address@hidden> wrote:
>> 
>> On 08.07.2013, at 15:08, Peter Maydell wrote:
>> 
>>> On 8 July 2013 13:59, Alexander Graf <address@hidden> wrote:
>>>> On 08.07.2013, at 14:57, Alexander Graf wrote:
>>>>> On 27.06.2013, at 15:04, Peter Maydell wrote:
>>>>>> The basic idea is that the board instantiates some transports,
>>>>> 
>>>>> I really dislike that idea. Couldn't you also create a new
>>>>> bus for your vexpress platform and add a virtio-mmio-vexpress
>>>>> device that automatically allocates an interrupt from the main
>>>>> PIC on instantiation? That way you could create transports
>>>>> using -device.
>>> 
>>> This doesn't seem to gain anything except that the user has
>>> to use -device twice rather than once.
>> 
>> Yes, it does. You can have other devices than just virtio ones
>> on those IRQ lines. Assigned devices for example.
> 
> Now I'm completely confused. Why would assigned devices
> have anything to do with this? Can you explain in more
> detail, because I don't really see what you're suggesting?

The only missing link we have to create any device using -device on the command 
line is the IRQ line enumeration. If we can allocate IRQ lines automatically, 
we can put any command line given -device onto our main system bus that is 
non-pci, non-isa.

So if we want to ever support VFIO for platform devices, the user will want to 
pass -device vfio-ahci,foo=bar on the command line to assign an AHCI device. 
The only infrastructure blocker we have for that today is the IRQ allocation. 
Maybe we could even try to be as smart as putting the MMIO regions into guest 
address space intelligently automatically.

So if we need all that logic anyway, I don't think it's a good idea to go into 
the wrong direction with virtio-mmio which has the exact same requirements.

> 
>>> As far as I know there's no way to find out at board
>>> construction how many virtio devices you might want.
>> 
>> Hence you shouldn't create them :).
> 
> You've got to create them sometime...

Yes, when the user says he wants them.


Alex