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

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.


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

It could even be a new BAR, which gives even more freedom to allocate
the fields.


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.


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