Nouveau - Nov 2025 - [PATCH] gpu: Move DRM buddy allocator one level up

On Tue, 25 Nov 2025 at 18:11, Christian K?nig <christian.koenig at
amd.com> wrote:
>
> On 11/25/25 08:59, John Hubbard wrote:
> > On 11/24/25 11:54 PM, Christian K?nig wrote:
> >> On 11/25/25 08:49, Dave Airlie wrote:
> >>> On Tue, 25 Nov 2025 at 17:45, Christian K?nig
<christian.koenig at amd.com> wrote:
> > ...
> >> My question is why exactly is nova separated into nova-core and
nova-drm? That doesn't seem to be necessary in the first place.
> >>
> > The idea is that nova-core allows building up a separate software
stack for
> > VFIO, without pulling in any DRM-specific code that a hypervisor (for
example)
> > wouldn't need. That makes for a smaller, more security-auditable
set of code
> > for that case.
>
> Well that is the same argument used by some AMD team to maintain a separate
out of tree hypervisor for nearly a decade.
>
> Additional to that the same argument has also been used to justify the KFD
node as alternative API to DRM for compute.
>
> Both cases have proven to be extremely bad ideas.
>
> Background is that except for all the legacy stuff the DRM API is actually
very well thought through and it is actually quite hard to come up with
something similarly well.
>
Well you just answered your own question, why is AMD maintaining GIM
instead of solving this upstream with a split model? the nova-core/drm
split would be perfect for GIM.

kfd was a terrible idea, and we don't intend to offer userspace
multiple APIs with nova, nova-drm will be the primary userspace API
provider. nova-core will not provide userspace API, it will provide an
API to nova-drm and an API to the vgpu driver which will provide it's
own userspace API without graphics or compute, just enough to setup
VFs.

Dave.

On 11/25/25 10:08, Dave Airlie wrote:
> On Tue, 25 Nov 2025 at 18:11, Christian K?nig <christian.koenig at
amd.com> wrote:
>>
>> On 11/25/25 08:59, John Hubbard wrote:
>>> On 11/24/25 11:54 PM, Christian K?nig wrote:
>>>> On 11/25/25 08:49, Dave Airlie wrote:
>>>>> On Tue, 25 Nov 2025 at 17:45, Christian K?nig
<christian.koenig at amd.com> wrote:
>>> ...
>>>> My question is why exactly is nova separated into nova-core and
nova-drm? That doesn't seem to be necessary in the first place.
>>>>
>>> The idea is that nova-core allows building up a separate software
stack for
>>> VFIO, without pulling in any DRM-specific code that a hypervisor
(for example)
>>> wouldn't need. That makes for a smaller, more
security-auditable set of code
>>> for that case.
>>
>> Well that is the same argument used by some AMD team to maintain a
separate out of tree hypervisor for nearly a decade.
>>
>> Additional to that the same argument has also been used to justify the
KFD node as alternative API to DRM for compute.
>>
>> Both cases have proven to be extremely bad ideas.
>>
>> Background is that except for all the legacy stuff the DRM API is
actually very well thought through and it is actually quite hard to come up with
something similarly well.
>>
> 
> Well you just answered your own question, why is AMD maintaining GIM
> instead of solving this upstream with a split model? the nova-core/drm
> split would be perfect for GIM.
No, it won't.

We have the requirement to work with GEM objects and DMA-buf file descriptors in
the hypervisor as well.

And my suspicion is that you end up with the same requirements in nova as well
in which case you end up interchanging handles with DRM as well.

We have seen the same for KFD and it turned out to be an absolutely horrible
interaction.
> kfd was a terrible idea, and we don't intend to offer userspace
> multiple APIs with nova, nova-drm will be the primary userspace API
> provider. nova-core will not provide userspace API, it will provide an
> API to nova-drm and an API to the vgpu driver which will provide it's
> own userspace API without graphics or compute, just enough to setup
> VFs.
Ok, then why do you need nova-core in the first place? E.g. where should be the
vgpu driver and what interface does it provide?

Christian.
> 
> Dave.