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

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

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


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