Re: [Qemu-devel] [Xen-devel] [PATCH 2/2] xen:i386:pc_piix: create isa bridge specific to IGD passthrough

["Chen, Tiejun" <tiejun.chen@intel.com>]

On 2014/8/31 16:58, Michael S. Tsirkin wrote:
> On Fri, Aug 29, 2014 at 09:28:50AM +0800, Chen, Tiejun wrote:
>>
>>
>> On 2014/8/28 8:56, Chen, Tiejun wrote:
>>>>>>>>> +     */
>>>>>>>>> +    dev = pci_create_simple(bus, PCI_DEVFN(0x1f, 0),
>>>>>>>>> +                            "xen-igd-passthrough-isa-bridge");
>>>>>>>>> +    if (dev) {
>>>>>>>>> +        r = xen_host_pci_device_get(&hdev, 0, 0, PCI_DEVFN(0x1f,
>>>>>>>>> 0), 0);
>>>>>>>>> +        if (!r) {
>>>>>>>>> +            pci_config_set_vendor_id(dev->config, hdev.vendor_id);
>>>>>>>>> +            pci_config_set_device_id(dev->config, hdev.device_id);
>>>>>>
>>>>>> Can you, instead, implement the reverse logic, probing
>>>>>> the card and supplying the correct device id for PCH?
>>>>>>
>>>>>
>>>>> Here what is your so-called reverse logic as I already asked you
>>>>> previously? Do you mean I should list all PCHs with a combo illustrated
>>>>> with the vendor/device id in advance? Then look up if we can find a
>>>>
>>>> Michael,
>>>>
>>>
>>> Ping.
>>>
>>> Thanks
>>> Tiejun
>>>
>>>> Could you explain this exactly? Then I can try follow-up your idea ASAP
>>>> if this is necessary and possible.
>>
>> Michel,
>>
>> Could you give us some explanation for your "reverse logic" when you're
>> free?
>>
>> Thanks
>> Tiejun
>
> So future drivers will look at device ID for the card
> and figure out how things should work from there.
> Old drivers still poke at device id of the chipset for this,
> but maybe qemu can do what newer drivers do:
> look at the card and figure out what guest should do,
> then present the appropriate chipset id.
>
> This is based on what Jesse said:
> 	Practically speaking, we could probably assume specific GPU/PCH combos,
> 	as I don't think they're generally mixed across generations, though SNB
> 	and IVB did have compatible sockets, so there is the possibility of
> 	mixing CPT and PPT PCHs, but those are register identical as far as the
> 	graphics driver is concerned, so even that should be safe.
>

Michael,

Thanks for your explanation.

> so the idea is to have a reverse table mapping GPU back to PCH.
> Present to guest the ID that will let it assume the correct GPU.

I guess you mean we should create to maintain such a table:

[GPU Card: device_id(s), PCH: device_id]

Then each time, instead of exposing that real PCH device id directly, 
qemu first can get the real GPU device id to lookup this table to 
present a appropriate PCH's device id to the guest.

And looks here that appropriate PCH's device id is not always a that 
real PCH's device id. Right? If I'm wrong please correct me.

>
> the problem with these tables is they are hard to keep up to date

Yeah. But I think currently we can just start from some modern CPU such 
as HSW and BDW, then things could be easy.

Allen,

Any idea to this suggestion?

> as new hardware comes out, but as future hardware won't need
> these hacks, we shall be fine.

Yeah.

Thanks
Tiejun

>
>
>>>>
>>>> Thanks
>>>> Tiejun
>>>>
>>>>> matched PCH? If yes, what is that benefit you expect in passthrough
>>>>> case? Shouldn't we pass these info to VM directly in passthrough case?
>>>>>
>>>>> Thanks
>>>>> Tiejun
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>