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

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.
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?


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