Re: VGA Passthrough

[BALATON Zoltan]

On Fri, 20 Mar 2020, Jd Lyons wrote:
> I’m using a version of Openbios that has the ATI code rom word patches.
>
> The situation with PCI Passthrough is complicated, I’m using a PCI-E to 
> PCI bridge, and I’m not sure how a VGA card needs to access it’s PCI 
> bridge. With a PCI Firewire card in the same bridge, everything works as 
> expected in qemu-system-ppc via PCI Passthrough.
>
> However, there maybe something about VGA Passthrough that requires some bridge 
> functions?
>
> VFIO-PCI doesn’t support bridges, so their is no way to pass the bridge/adapter 
> to Qemu.
>
> Here is the host MMUIO group:
>
> IOMMU Group 21 24:00.0 PCI bridge [0604]: ASMedia Technology Inc. ASM1083/1085 
> PCIe to PCI Bridge [1b21:1080] (rev 04)
> IOMMU Group 21 25:00.0 VGA compatible controller [0300]: Advanced Micro 
> Devices, Inc. [AMD/ATI] Rage Fury/Xpert 128/Xpert 2000 AGP 2x [1002:5246]

Wait, you have an AGP card in a PCI bridge? Should this even work? I think 
accessing the system memory for bus master transfers work differently on 
PCI and AGP with AGP using GART and ATI cards using similar on PCI but 
probably not compatible. This would at least confuse drivers to find an 
AGP card on PCI bus and probably try to use GART which might not work 
(although I don't know it well so I might be wrong).

I also don't know vfio and passthrough but I've read something that whole 
iommu groups should be passed through? If that's true you may need to pass 
it together with the bridge but that likely won't work on g3beige and 
mac99 becuase those don't have PCIe so guests won't have drivers and can't 
use the bridge.

So I'm not sure what you're doing is even possible, some confirmation from 
someone who knows more about pass through would be welcome before trying 
to go deeper debugging it.

Regards,
BALATON Zoltan