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

On 2015/2/4 18:41, Ian Campbell wrote:
On Wed, 2015-02-04 at 09:34 +0800, Chen, Tiejun wrote:

       "-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.

Neither '-gfx_passthru' nor 'igd-passthrou' exists in any version of
qemu upstream. As I mentioned previously, just now we're starting to
support this in qemu upstream :)

But you known, on Xen side we have two qemu versions,
qemu-xen-traditional and qemu-xen. Although '-gfx_passthru' is adopted
in both two versions, just qemu-xen-traditional supports IGD passthrough
completely. For qemu-xen, we just have this term definition but without
that IGD passthrough feature support actually.

I'm afraid I don't follow, you seem to be simultaneously saying that
qemu-xen both does and does not support -gfx_passthru, which cannot be
right. I think from the following paragraph that what you mean is that
upstream qemu-xen has no support for any kind of -gfx_passthru command
line option in any form in any existing version, including the git tree.
Is that correct?


(for the purposes of this conversation qemu-traditional is not of


And now we're trying to support IGD passthrough in qemu stream. This
mean we should set 'device_model_version="qemu-xen", right? Then libxl
still pass '-gfx_passthru' to qemu upstream. But when I post other
stuffs to qemu upstream community to support IGD passthrough. Gerd
thought "-machine xxx,igd-passthru=on" is better than '-gfx_passthru'.
So finally you see this change in Xen/tools/libxl.

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

Actually gfx_passthrou is introduced to just work for IGD passthrough
since something specific to IGD is tricky, so we have to need such a
flag to handle this precisely, like its fixed at 00:02.0, and expose
some ISA bridge PCI config info and even host bridge PCI config info.

So I don't thing other cards need this.

If one type VGA device needs these sorts of workaround it is not
inconceivable that some other one will also need workarounds in the

Indeed this is not something workaround, and I think in any type of VGA devices, we'd like to diminish this sort of thing gradually, right?

This mightn't come true in real world :)

Even if you don't consider non-IGD, what about the possibility of a
future IGD device which needs different (or additional, or fewer)

As far as I know we're trying to drop off those dependencies on ISA bridge and host bridge in our IGD's roadmap. Because in any pass through cases, theoretically we should access those resources dedicated to that device.

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.

Based on the above explanation, something should be done before we
detect to construct that real card , so its difficulty to achieve this
goal currently.

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?

Sorry I don't understand what you mean here.

"gfx_passthru" is a generically named option, but it is being
implemented in an IGD specific way. We need to consider the possibility
of other graphics devices needing special handling in the future and
plan accordingly such that we can try and maintain our API guarantees
when this happens.


I think there are three ways to achieve that:

       * Make the libxl/xl option something which is not generic e.g.
       * Add a second option to allow the user to configure the kind of
         graphics device being passed thru (e.g. gfx_passthru=1,
         passthru_device="igd"), or combine the two by making the
         gfx_passthru option a string instead of a boolean.

It makes more sense but this mean we're going to change that existing rule in qemu-traditional. But here I guess we shouldn't consider that case.

       * Make libxl detect the type of graphics device somehow and
         therefore automatically determine when gfx_passthru=1 =>

This way confounds me all. Can libxl detect the graphics device *before* we intend to pass a parameter to active qemu?

  Currently, we have to set
something as follows,


This always works for qemu-xen-traditional.

But you should know '00:02.0' doesn't mean we are passing IGD through.

But by looking at the device 00:02.0 (e.g. its PCI vendor and device ID
and other properties) we can unambiguously determine if it is an IGD
device or not, can't we?

Again, like what I said above, I'm not sure if its possible in my case. If I'm wrong please correct me.

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?

Are you saying two IGDs?

Yes, or any combination of two cards, perhaps from different vendors
(AIUI some laptops have this with IGD and Nvidia or ATI?).

One IGD and multiple other type of Graphic display cards can coexist.

  Its not possible since as I said above, IGD is
tricky because it depends on something from ISA bridge and host bridge.
So we can't provide two or more different setting configurations to own
more IGDs just in one platform.

This is because IGD must be a "primary" VGA device? I understand that

No. I mean ISA bridge and host bridge just provide one set of IGD resource so its difficult to configure two or more IGDs.

there can only be one of those in a system, but I think it is possible
to have multiple secondary VGA devices or different types in one system.

What I'm saying is, its impossible to own two same IGDs in our current platform :)


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