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 2014/9/1 14:05, Michael S. Tsirkin wrote:
> On Mon, Sep 01, 2014 at 10:50:37AM +0800, Chen, Tiejun wrote:
>> 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.
>
> Exactly: we don't really care what the PCH ID is - we only need it
> for the guest driver to do the right thing.

Agreed.

I need to ask Allen to check if one given GPU card device id is always 
corresponding to one given PCH on both HSW and BDW currently. If yes, I 
can do this quickly. Otherwise I need some time to establish that sort 
of connection. 	

Thanks
Tiejun

>
>>>
>>> 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
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>
>
>