Re: [PATCH drm-next 00/14] [RFC] DRM GPUVA Manager & Nouveau VM_BIND UAPI

[Dave Airlie <airlied@gmail.com>]

On Thu, 19 Jan 2023 at 02:54, Alex Deucher <alexdeucher@gmail.com> wrote:
>
> On Wed, Jan 18, 2023 at 11:50 AM Danilo Krummrich <dakr@redhat.com> wrote:
> >
> >
> >
> > On 1/18/23 17:30, Alex Deucher wrote:
> > > On Wed, Jan 18, 2023 at 11:19 AM Danilo Krummrich <dakr@redhat.com> wrote:
> > >>
> > >> On 1/18/23 16:37, Christian König wrote:
> > >>> Am 18.01.23 um 16:34 schrieb Danilo Krummrich:
> > >>>> Hi Christian,
> > >>>>
> > >>>> On 1/18/23 09:53, Christian König wrote:
> > >>>>> Am 18.01.23 um 07:12 schrieb Danilo Krummrich:
> > >>>>>> This patch series provides a new UAPI for the Nouveau driver in
> > >>>>>> order to
> > >>>>>> support Vulkan features, such as sparse bindings and sparse residency.
> > >>>>>>
> > >>>>>> Furthermore, with the DRM GPUVA manager it provides a new DRM core
> > >>>>>> feature to
> > >>>>>> keep track of GPU virtual address (VA) mappings in a more generic way.
> > >>>>>>
> > >>>>>> The DRM GPUVA manager is indented to help drivers implement
> > >>>>>> userspace-manageable
> > >>>>>> GPU VA spaces in reference to the Vulkan API. In order to achieve
> > >>>>>> this goal it
> > >>>>>> serves the following purposes in this context.
> > >>>>>>
> > >>>>>>       1) Provide a dedicated range allocator to track GPU VA
> > >>>>>> allocations and
> > >>>>>>          mappings, making use of the drm_mm range allocator.
> > >>>>>
> > >>>>> This means that the ranges are allocated by the kernel? If yes that's
> > >>>>> a really really bad idea.
> > >>>>
> > >>>> No, it's just for keeping track of the ranges userspace has allocated.
> > >>>
> > >>> Ok, that makes more sense.
> > >>>
> > >>> So basically you have an IOCTL which asks kernel for a free range? Or
> > >>> what exactly is the drm_mm used for here?
> > >>
> > >> Not even that, userspace provides both the base address and the range,
> > >> the kernel really just keeps track of things. Though, writing a UAPI on
> > >> top of the GPUVA manager asking for a free range instead would be
> > >> possible by just adding the corresponding wrapper functions to get a
> > >> free hole.
> > >>
> > >> Currently, and that's what I think I read out of your question, the main
> > >> benefit of using drm_mm over simply stuffing the entries into a list or
> > >> something boils down to easier collision detection and iterating
> > >> sub-ranges of the whole VA space.
> > >
> > > Why not just do this in userspace?  We have a range manager in
> > > libdrm_amdgpu that you could lift out into libdrm or some other
> > > helper.
> >
> > The kernel still needs to keep track of the mappings within the various
> > VA spaces, e.g. it silently needs to unmap mappings that are backed by
> > BOs that get evicted and remap them once they're validated (or swapped
> > back in).
>
> Ok, you are just using this for maintaining the GPU VM space in the kernel.
>

Yes the idea behind having common code wrapping drm_mm for this is to
allow us to make the rules consistent across drivers.

Userspace (generally Vulkan, some compute) has interfaces that pretty
much dictate a lot of how VMA tracking works, esp around lifetimes,
sparse mappings and splitting/merging underlying page tables, I'd
really like this to be more consistent across drivers, because already
I think we've seen with freedreno some divergence from amdgpu and we
also have i915/xe to deal with. I'd like to at least have one place
that we can say this is how it should work, since this is something
that *should* be consistent across drivers mostly, as it is more about
how the uapi is exposed.

Dave.