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

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

On 2015/2/5 17:52, Ian Campbell wrote:
> On Thu, 2015-02-05 at 09:22 +0800, Chen, Tiejun wrote:
> > 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 :)
> It's not really something we can control, the h/w guys will do what they
> do, including integrating on-board graphics tightly with the N/S-bridges
> etc.
Yeah.

> > > I think there are three ways to achieve that:
> > >
> > >         * Make the libxl/xl option something which is not generic e.g.
> > >           igd_passthru=1
> > >         * 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.
> qemu-trad is frozen so we wouldn't be adding new features such as
> support for new graphics passthru devices, so we can ignore it's
> deficiencies in this area and just improve things in the qemu-xen case.
> (we may want to add a compat handling for any new option we add to libxl
> to translate to -gfx_passthru, but that's about it)
Understood.

> > >         * Make libxl detect the type of graphics device somehow and
> > >           therefore automatically determine when gfx_passthru=1 =>
> > >           igd-passthru
> > This way confounds me all. Can libxl detect the graphics device *before*
> > we intend to pass a parameter to active qemu?
> We know the BDF of all devices which we are going to pass to the guest
> and we can observe various properties of that device
> via /sys/bus/pci/devices/0000:B:D:F.
So sounds what you're saying is Xen already have this sort of example in 
libxl.
> > > >    Currently, we have to set
> > > > something as follows,
> > > >
> > > > gfx_passthru=1
> > > > pci=["00:02.0"]
> > > >
> > > > 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.
> Is my reply above sufficient?
Yes, I can understand what you mean but I need to take close look at 
exactly what should be done in libxl :)
In high level, this way may come out as follows,

#1 libxl parse 'pci=[]' to get SBDF
#2 Scan SBDF by accessing sysfs to get vendor/device IDs.
#3 If this pair of IDs is identified to our target device, IGD, 
"igd-passthru" would be delivered to qemu.
Right?

> > > > > 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.
> ... but if they both need special handling then we need a way to
> communicate that.
>
> > > >    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 :)
> Understood, but systems needn't be homogeneous wrt graphics devices.
Ian,

Thanks for your kind discussion.

Tiejun

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