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Re: [Xen-devel] [v2][PATCH] libxl: add one machine property to support IGD GFX passthrough

On Tue, 2015-02-03 at 09:04 +0800, Chen, Tiejun wrote:
> On 2015/2/2 20:54, Ian Jackson wrote:
> > Wei Liu writes ("Re: [v2][PATCH] libxl: add one machine property to support 
> > IGD GFX passthrough"):
> >> On Mon, Feb 02, 2015 at 09:17:23AM +0800, Tiejun Chen wrote:
> >>> When we're working to support IGD GFX passthrough with qemu
> >>> upstream, instead of "-gfx_passthru" we'd like to make that
> >>> a machine option, "-machine xxx,-igd-passthru=on". This need
> >>> to bring a change on tool side.
> > ...
> >> My suggestion has one premise -- if upstream QEMU has already released
> >> that -gfx_passthru option. If there is no "old one" (in upstream QEMU)
> >> at all, then there is nothing to keep and deprecate.
> >
> > I think the commit message of the xen.git commit should explain what
> > options are supported by which versions of qemu (including qemu
> > upstream's future plans).
> >
> > That would provide (a) something which summarises the communication
> > etc. with qemu upstream and can be checked with them if necessary and
> > (b) something against which the libxl changes can be easily judged.

> Sorry, looks I'm misleading this to everyone.
> Here I picked my reply from another email:
> Actually qemu upstream never own this option, '-gfx_passthru' at all. 
> This just exists alone in qemu-xen-traditional. So here I'm trying to 
> introduce a new stuff that doesn't clash anything in qemu upstream.
> So I guess I should rephrase this as follows:
>      libxl: add one machine property to support IGD GFX passthrough
>      When we're working to support IGD GFX passthrough with qemu
>      upstream, we'd like to introduce a machine option,

Has this new option been acked/accepted into the upstream qemu code base
yet? I think there should also be a reference to the relevant qemu.git
changeset as well as to any useful conversations about it.

>      "-machine xxx,igd-passthru=on", to enable/disable that feature.
>      And we also remove that old option, "-gfx_passthru", just from
>      the case of LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN since actually
>      no any qemu stream version really need or use that.
                   ^up ?

What happens if you pass this new option to an older version of qemu
upstream? I suppose it doesn't fail any worse than passing -gfx_passthru
would have done.

I have one more general concern, which is that hardcoding igd-passthru
here may make it harder to support gfx_passthru of other cards in the

Somehow something in the stack needs to either detect or be told which
kind of card to passthrough. Automatic detection would be preferable,
but maybe being told by the user is the only possibility.

Is there any way, given gfx_passthru as a boolean that libxl can
automatically figure out that IGD passthru is what is actually desired
-- e.g. by scanning the set of PCI devices given to the guest perhaps?

If not then that _might_ suggest we should deprecate the gdx_passthru
option at the libxl interface and switch to something more expressive,
such as an Enumeration of card types (with a singleton of IGD right
now), but I'm not really very familiar with passthru nor the qemu side
of this.

What happens if you try to pass two different GFX cards to a guest?


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