Re: [Xen-devel] [Qemu-devel] [PATCH 2/2] xen:i386:pc_piix: create isa bridge specific to IGD passthrough

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

On 2014/8/31 16:58, Michael S. Tsirkin wrote:
> On Fri, Aug 29, 2014 at 09:28:50AM +0800, Chen, Tiejun wrote:
> > On 2014/8/28 8:56, Chen, Tiejun wrote:
> > > > > > > > > +     */
> > > > > > > > > +    dev = pci_create_simple(bus, PCI_DEVFN(0x1f, 0),
> > > > > > > > > +                            "xen-igd-passthrough-isa-bridge");
> > > > > > > > > +    if (dev) {
> > > > > > > > > +        r = xen_host_pci_device_get(&hdev, 0, 0, PCI_DEVFN(0x1f,
> > > > > > > > > 0), 0);
> > > > > > > > > +        if (!r) {
> > > > > > > > > +            pci_config_set_vendor_id(dev->config, hdev.vendor_id);
> > > > > > > > > +            pci_config_set_device_id(dev->config, hdev.device_id);
> > > > > > Can you, instead, implement the reverse logic, probing
> > > > > > the card and supplying the correct device id for PCH?
> > > > > Here what is your so-called reverse logic as I already asked you
> > > > > previously? Do you mean I should list all PCHs with a combo illustrated
> > > > > with the vendor/device id in advance? Then look up if we can find a
> > > > Michael,
> > > Ping.
> > >
> > > Thanks
> > > Tiejun
> > >
> > > > Could you explain this exactly? Then I can try follow-up your idea ASAP
> > > > if this is necessary and possible.
> > Michel,
> >
> > Could you give us some explanation for your "reverse logic" when you're
> > free?
> >
> > Thanks
> > Tiejun
> So future drivers will look at device ID for the card
> and figure out how things should work from there.
> Old drivers still poke at device id of the chipset for this,
> but maybe qemu can do what newer drivers do:
> look at the card and figure out what guest should do,
> then present the appropriate chipset id.
>
> This is based on what Jesse said:
>         Practically speaking, we could probably assume specific GPU/PCH combos,
>         as I don't think they're generally mixed across generations, though SNB
>         and IVB did have compatible sockets, so there is the possibility of
>         mixing CPT and PPT PCHs, but those are register identical as far as the
>         graphics driver is concerned, so even that should be safe.
Michael,

Thanks for your explanation.

> so the idea is to have a reverse table mapping GPU back to PCH.
> Present to guest the ID that will let it assume the correct GPU.
I guess you mean we should create to maintain such a table:

[GPU Card: device_id(s), PCH: device_id]

Then each time, instead of exposing that real PCH device id directly, 
qemu first can get the real GPU device id to lookup this table to 
present a appropriate PCH's device id to the guest.
And looks here that appropriate PCH's device id is not always a that 
real PCH's device id. Right? If I'm wrong please correct me.
> the problem with these tables is they are hard to keep up to date
Yeah. But I think currently we can just start from some modern CPU such 
as HSW and BDW, then things could be easy.
Allen,

Any idea to this suggestion?

> as new hardware comes out, but as future hardware won't need
> these hacks, we shall be fine.
Yeah.

Thanks
Tiejun

> > > > Thanks
> > > > Tiejun
> > > >
> > > > > matched PCH? If yes, what is that benefit you expect in passthrough
> > > > > case? Shouldn't we pass these info to VM directly in passthrough case?
> > > > >
> > > > > Thanks
> > > > > Tiejun
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