Re: [PATCH v5 05/11] drm: nova: Add an info ioctl

[Alistair Popple <apopple@nvidia.com>]

On 2026-09-01 at 14:53 +1000, Dave Airlie <airlied@gmail.com> wrote...
> On Fri, 28 Aug 2026 at 13:36, Alistair Popple <apopple@nvidia.com> wrote:
> >
> > Add an extensible info ioctl and use it to report basic GPU information.
> > One of the first things userspace needs to know about a GPU is its
> > architecture and implementation, so add that as the first field in the
> > GPU info result.
> >
> > The ioctl selects an information type by ID and writes the result through
> > a sized userspace buffer. The kernel truncates results to the supplied
> > size, allowing information structures to grow and new logical information
> > groups to be added without introducing new ioctls.
> >
> >
> 
> This is a higher level for the future, I think we have two paths for
> getparam type ioctl.
> 
> Either we expose a big struct full of values with a add to the end
> mentality or we keep a long list of value pairs, and userspace passes
> in a list of value keys, and we return a list of the values.

There's already been a fair bit of back and forth on this on this series and
this is where we ended up - a bunch of info ioctls returning big structs full
of values.

I did wonder if io_uring could be used by user-space to efficiently pass a list
of value keys as that's trivial enough to add to drm[1] but perf wise didn't
achieve much over just calling lots of ioctls, at least when I tested with a
mock key/value type interface.

[1] - https://lists.freedesktop.org/archives/dri-devel/2025-September/524469.html

> I'm trying to think ahead towards native virtio context support, and I
> think having the sequence,
> 
> open
> get_all_infos
> create vm/channel
> 
> might have the most optimal ioctl count but also let power management
> avoid booting the GPU on every open if we never create a vm/channel.

Power management is a good point - thus far all the info is cached in nova-core
anyway so there shouldn't be much impact there regardless of what interface
we choose.

Thanks.

 - Alistair