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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Mon, Sep 01, 2014 at 10:50:37AM +0800, Chen, Tiejun wrote:
> 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.

Exactly: we don't really care what the PCH ID is - we only need it
for the guest driver to do the right thing.

> >
> >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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