Re: [Qemu-devel] [v5][PATCH 0/5] xen: add Intel IGD passthrough support

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

On 2014/6/25 17:09, Michael S. Tsirkin wrote:
> On Wed, Jun 25, 2014 at 04:55:25PM +0800, Chen, Tiejun wrote:
>> On 2014/6/25 16:48, Michael S. Tsirkin wrote:
>>> On Wed, Jun 25, 2014 at 10:39:43AM +0200, Paolo Bonzini wrote:
>>>> Il 25/06/2014 10:31, Michael S. Tsirkin ha scritto:
>>>>> It might be possible to move the Q35 bridge elsewhere.
>>>>> seabios doesn't care where the host bridge is.
>>>>> ACPI tables in QEMU can be adjusted.
>>>>
>>>> But why?  It's always in 00:1f.0 on real hardware.  If the i915 driver wants
>>>> to run under virtual machines, it should stop acting as if it knows what the
>>>> whole machine looks like.
>>>>
>>>> Paolo
>>>
>>> If guest driver can be fixed that seems ideal.
>>> I don't know how it works but presumably you guys do?
>>
>> Paolo prefer we need to fix this in the driver like:
>>
>> "
>>>>> 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.
>> "
>>
>> Thanks
>> Tiejun
>
> Adding a vendor-specific capability or BAR
> in an existing device is painful - hard to find
> free space for it.

Yes, this is a potential risk as well since we can't guarantee current 
free space is always valid for ever.

> We could add a dummy device in PCI or ACPI, but
> driver should really look for it using device/vendor id,
> not at a specific address.
> What exactly is it that the driver wants to know?
>

The i915 driver need to use the vendor/device ids to get what pch type 
is running, then go different paths.

Thanks
Tiejun