Re: [Qemu-devel] [PATCH RFC 0/3] basic support for composing sysbus devices

[Blue Swirl]

On Tue, Jun 14, 2011 at 4:21 PM, Anthony Liguori <address@hidden> wrote:
> On 06/13/2011 03:59 PM, Blue Swirl wrote:
>>
>> On Sun, Jun 12, 2011 at 10:21 PM, Anthony Liguori<address@hidden>
>>  wrote:
>>>
>>> On 06/12/2011 12:12 PM, Avi Kivity wrote:
>>>>
>>>> On 06/10/2011 06:43 PM, Anthony Liguori wrote:
>>>>>
>>>>>> What exactly is so very wrong about buses that they need to die?
>>>>>
>>>>> They force a device tree. The device model shouldn't be a tree, but a
>>>>> directed graph.
>>>>
>>>> Right. As an example, you configure PCI interrupt routing and the memory
>>>> controller by writing to a PCI device, which logically doesn't have
>>>> access to any of this stuff if it's behind the PCI bus.
>>>>
>>>> However, I don't think buses should die. They should be available as an
>>>> easy way to model the devices that do follow the rules. But we should
>>>> also expose everything else for the exceptional cases.
>>>>
>>>>> It's perfectly fine to have a type called PCIBus that I440FX extends,
>>>>> but qdev shouldn't have explicit knowledge of something called a "bus"
>>>>> IMHO. Doing this forces a limited mechanism of connecting devices
>>>>> because it creates an artificial tree (by implying a parent/child
>>>>> relationship). It makes composition difficult if not impossible.
>>>>
>>>> I think qdev buses are useful as long as they don't enforce their
>>>> interfaces. That is, a qdev that is a child of a qbus has access to the
>>>> qbus's interfaces, but also access to other stuff.
>>>
>>> I see two independent data structures.  The first is the "instantiation
>>> tree".
>>>
>>> The instantiation tree may look like this:
>>>
>>> +-- i440fx
>>> |  |
>>> |  +-- PIIX3
>>> |  |  |
>>> |  |  +-- mc146818a
>>> |  |  +-- uart
>>> |  |  +-- DMA
>>> |  |  +-- keyboard controller
>>> |  |  +-- (remaining platform ISA devices
>>> |  |
>>> |  +-- UHCI USB controller
>>> |  +-- IDE controller
>>> |
>>> +-- e1000
>>> +-- cirrus-vga
>>> +-- virtio-balloon-pci
>>> +-- IDE disk0
>>>
>>> Instantiating i440fx makes a bunch of default stuff.  This is
>>> composition.
>>>  Everything else requires explicit instantiation.  This is, strictly
>>> speaking, the parent/child relationships.  If you destroy i440fx, all of
>>> it's children have to also go away (by definition). Nothing about bus
>>> relationship is implied here.  Even if i440fx exposes a PCI bus, the
>>> PIIX3
>>> is a child of i440fx even though e1000 is not (even if they're both PCI
>>> devices).
>>
>> I actually like this slot idea in place of buses. But wouldn't there
>> be two classes of devices (or two APIs), slot devices and composable
>> devices?
>
> All devices have properties.  We have this today in qdev.  What's missing is
> to have a properties who's type is a socket for another device.  We really
> want to be able to do:
>
> static DeviceInfo i440fx_info = {
>    .name = "i440fx",
>    .props = (Property[]){
>       DEFINE_PROP_PLUG(I440FXState, piix3),
>       DEFINE_PROP_SOCKET(I440FXState, slot[0]),
>       DEFINE_PROP_SOCKET(I440FXState, slot[1]),
>       ...
>    },
> };
>
> Which suggests that we really need to move away from declarative device
> definitions.  It makes it hard to have variable numbers of properties.

I'd suppose the number of slots could be fixed. Then the declaration could be
      DEFINE_PROP_SOCKETS(I440FXState, slot, 32),

> In this case, piix3 would be defined as:
>
> struct I440FXState {
>    PIIX3 piix3;
>    PCIDevice slots[32];
> };
>
> Which suggests we need an initfn to do the following:
>
> void i440fx_initfn(...) {
>   qdev_init_inplace(&dev->piix3, "PIIX3");
>   dev->slot[1] = &dev->piix3->bus;
> }
>
> This gets hard to do well in C though.  I'm not sure how we could make
> DEFINE_PROP_PLUG/SOCKET type safe.

DEFINE_PROP_PCI_SOCKET()?