[vfio-users] radeon vfio bus information issue?

[globalgorrilla at fastmail.fm]

On 24 Oct 2016, at 10:23, Alex Williamson wrote:

> On Mon, 24 Oct 2016 10:20:08 -0700
> globalgorrilla at fastmail.fm wrote:
>
>> Hi,
>>
>> Pass-through works fine with the the same linux 4.8 VM and nouveau 
>> and a
>> Quadro K2200.
>>
>> Passing-through both a R7 260X and and a R9 290 I get the same kernel
>> oops:
>>
>> IP: [<ffffffffc029efd9>] drm_pcie_get_speed_cap_mask+0x39/0xf0 [drm]
>>
>> More here:
>> http://pastebin.com/Waysyk2e
>>
>> Both the AMD cards work fine if I pass them through to a Windows VM.
>>
>> I'm supposing this might likely be then an issue with Radeon.
>>
>> However the oops makes me think perhaps it's breaking on unexpected 
>> PCIE
>> information?
>>
>> Is it possible there some missing information in that radeon is 
>> looking
>> for that's missing? What PCIE information is passed through by vfio
>> (lspci from both host and vm also in the pastebin)?
>>
>> Anybody using radeon with passthrough AMD cards and seen this?
>
> Radeon with a Linux guest is actually one of the few cases where you
> need to be running a Q35 machine with the GPU placed behind a PCIe
> downstream/root port.  The code blindly assumes that an upstream PCIe
> bridge is present and tries to poke registers on it.

Bingo.

I'll have to clone and try with that.

Alex, do you know if is enough to just simply passthrough the device but 
with Q35? Or is a custom topology needed?

Thanks!