Re: [Qemu-devel] [Xen-devel] [v2][PATCH] libxl: add one machine property to support IGD GFX passthrough

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

On 2015/2/5 17:52, Ian Campbell wrote:
> On Thu, 2015-02-05 at 09:22 +0800, Chen, Tiejun wrote:
>> Indeed this is not something workaround, and I think in any type of VGA
>> devices, we'd like to diminish this sort of thing gradually, right?
>>
>> This mightn't come true in real world :)
>
> It's not really something we can control, the h/w guys will do what they
> do, including integrating on-board graphics tightly with the N/S-bridges
> etc.

Yeah.

>
>>>
>>> I think there are three ways to achieve that:
>>>
>>>         * Make the libxl/xl option something which is not generic e.g.
>>>           igd_passthru=1
>>>         * Add a second option to allow the user to configure the kind of
>>>           graphics device being passed thru (e.g. gfx_passthru=1,
>>>           passthru_device="igd"), or combine the two by making the
>>>           gfx_passthru option a string instead of a boolean.
>>
>> It makes more sense but this mean we're going to change that existing
>> rule in qemu-traditional. But here I guess we shouldn't consider that case.
>
> qemu-trad is frozen so we wouldn't be adding new features such as
> support for new graphics passthru devices, so we can ignore it's
> deficiencies in this area and just improve things in the qemu-xen case.
> (we may want to add a compat handling for any new option we add to libxl
> to translate to -gfx_passthru, but that's about it)

Understood.

>
>>>         * Make libxl detect the type of graphics device somehow and
>>>           therefore automatically determine when gfx_passthru=1 =>
>>>           igd-passthru
>>
>> This way confounds me all. Can libxl detect the graphics device *before*
>> we intend to pass a parameter to active qemu?
>
> We know the BDF of all devices which we are going to pass to the guest
> and we can observe various properties of that device
> via /sys/bus/pci/devices/0000:B:D:F.

So sounds what you're saying is Xen already have this sort of example in 
libxl.

>
>>
>>>
>>>>    Currently, we have to set
>>>> something as follows,
>>>>
>>>> gfx_passthru=1
>>>> pci=["00:02.0"]
>>>>
>>>> This always works for qemu-xen-traditional.
>>>>
>>>> But you should know '00:02.0' doesn't mean we are passing IGD through.
>>>
>>> But by looking at the device 00:02.0 (e.g. its PCI vendor and device ID
>>> and other properties) we can unambiguously determine if it is an IGD
>>> device or not, can't we?
>>
>> Again, like what I said above, I'm not sure if its possible in my case.
>> If I'm wrong please correct me.
>
> Is my reply above sufficient?

Yes, I can understand what you mean but I need to take close look at 
exactly what should be done in libxl :)

In high level, this way may come out as follows,

#1 libxl parse 'pci=[]' to get SBDF
#2 Scan SBDF by accessing sysfs to get vendor/device IDs.
#3 If this pair of IDs is identified to our target device, IGD, 
"igd-passthru" would be delivered to qemu.

Right?

>
>>>>> If not then that _might_ suggest we should deprecate the gdx_passthru
>>>>> option at the libxl interface and switch to something more expressive,
>>>>> such as an Enumeration of card types (with a singleton of IGD right
>>>>> now), but I'm not really very familiar with passthru nor the qemu side
>>>>> of this.
>>>>>
>>>>> What happens if you try to pass two different GFX cards to a guest?
>>>>>
>>>>
>>>> Are you saying two IGDs?
>>>
>>> Yes, or any combination of two cards, perhaps from different vendors
>>> (AIUI some laptops have this with IGD and Nvidia or ATI?).
>>
>> One IGD and multiple other type of Graphic display cards can coexist.
>
> ... but if they both need special handling then we need a way to
> communicate that.
>
>>>>    Its not possible since as I said above, IGD is
>>>> tricky because it depends on something from ISA bridge and host bridge.
>>>> So we can't provide two or more different setting configurations to own
>>>> more IGDs just in one platform.
>>>
>>> This is because IGD must be a "primary" VGA device? I understand that
>>
>> No. I mean ISA bridge and host bridge just provide one set of IGD
>> resource so its difficult to configure two or more IGDs.
>>
>>> there can only be one of those in a system, but I think it is possible
>>> to have multiple secondary VGA devices or different types in one system.
>>>
>>
>> What I'm saying is, its impossible to own two same IGDs in our current
>> platform :)
>
> Understood, but systems needn't be homogeneous wrt graphics devices.
>

Ian,

Thanks for your kind discussion.

Tiejun