Re: [Qemu-devel] [RFC][PATCH] gpu:drm:i915:intel_detect_pch: back to check devfn instead of check class type

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

On 2014/6/25 15:55, Paolo Bonzini wrote:
> Il 25/06/2014 09:34, Chen, Tiejun ha scritto:
>> On 2014/6/25 14:48, Paolo Bonzini wrote:
>>> Second problem.  Your IGD passthrough code currently works with QEMU's
>>> PIIX4-based machine.  But what happens if you try to extend it, so that
>>
>> Yes, current xen machine, xenpv, is based on pii4, and also I don't
>> known if we will plan to migrate to q35 or others. So its hard to
>> further say more now.
>>
>>> it works with QEMU's ICH9-based machine?  That's a more modern machine
>>> that has a PCIe chipset and hence has its ISA bridge at 00:1f.0.  Now
>>
>> But even in this case, could we set the real vendor/device ids for that
>> ISA bridge at 00:1f.0? If not, what's broken?
>
> The config space layout changes for different vendor/device ids, so the
> guest firmware only works if you have the right vendor/device id.

Paolo,

After I discuss internal, we think even we just set the real 
vendor/device ids to this ISA bridge at 00:1f.0, guest firmware should 
still work well with these pair of real vendor/device ids.

So if you think something would conflict or be broken, could you tell us 
what's exactly that? Then we will double check.

Thanks
Tiejun

>
>>> It is only slightly better, but the right solution is to fix the driver.
>>>   There is absolutely zero reason why a graphics driver should know
>>> about the vendor/device ids of the PCH.
>>
>> This means we have to fix this both on Linux and Windows but I'm not
>> sure if this is feasible to us.
>
> You have to do it if you want this feature in QEMU in a future-proof way.
>
> You _can_ provide the ugly PIIX4-specific hack as a compatibility
> fallback (and this patch is okay to make the compatibility fallback less
> hacky).  However, I don't think QEMU should accept the patch for IGD
> passthrough unless Intel is willing to make drivers
> virtualization-friendly.  Once you assign the IGD, it is not that
> integrated anymore and the drivers must take that into account.
>
> It is worthwhile pointing out that neither AMD nor nVidia need any of this.
>
>>> The right way could be to make QEMU add a vendor-specific capability to
>>> the video device. The driver can probe for that capability before
>>
>> Do you mean we can pick two unused offsets in the configuration space of
>> the video device as a vendor-specific capability to hold the
>> vendor/device ids of the PCH?
>
> Yes, either that or add a new capability (which lets you choose the
> offsets more freely).
>
> If the IGD driver needs config space fields of the MCH, those fields
> could also be mirrored in the new capability.  QEMU would forward them
> automatically.
>
> It could even be a new BAR, which gives even more freedom to allocate
> the fields.
>
>>> looking at the PCI bus.  QEMU can add the capability to the list, it is
>>> easy because all accesses to the video device's configuration space trap
>>> to QEMU.  Then you do not need to add fake devices to the machine.
>>>
>>> In fact, it would be nice if Intel added such a capability on the next
>>> generation of integrated graphics, too.  On real hardware, ACPI or some
>>
>> Maybe, but even this would be implemented, shouldn't we need to be
>> compatible with those old generations?
>
> Yes.
>
> - old generation / old driver: use 00:1f.0 hack, only guaranteed to work
> on PIIX4-based virtual guest
>
> - old generation / new driver: use 00:1f.0 hack on real hardware, use
> capability on 00:02.0 on virtual guest, can work on PCIe virtual guest
>
> - new generation / old driver: doesn't exist
>
> - new generation / new driver: always use capability on 00:02.0, can
> work on PCIe virtual guest.
>
> Paolo
>
>