[PATCH v2 0/4] rust: Add support for reserving of ranges of IDs

[Eliot Courtney]

Add support for reserving of ranges of IDs, with a usage in nova-core
for channel IDs. This entails adding bindings for the C bitmap
API for ranges of bits, then users of that in `IdPool`, and finally a
user of `IdPool` in nova-core, `ChannelIdPool`.

Channel ID tracking is needed for allotting ranges of channel IDs to
vGPU guests, and later for regular host channel ID reservation.
nova-core needs allocation of a contiguous sequence of IDs with a
specific length and sometimes a specific alignment [1].

About the tradeoffs between different data structures:
- IDA/xarray do not support allocating a contiguous sequence of IDs
  (ida_alloc_range() allocates a single ID within a range, not a contiguous
  sequence).
- A maple tree works, but is not as good a fit. The ID space is small
  (limited to 2048) and aligned allocation needs an alloc_range()+erase() retry
  loop (plus a Mutex around it, or new mas_empty_area() bindings) that
  essentially reimplements bitmap_find_next_zero_area(). See the maple tree
  version at [2]. For 2048 IDs a bitmap is also considerably faster and smaller
  [3].
- The bitmap API natively supports aligned contiguous area allocation
  (bitmap_find_next_zero_area()).

This is based on drm-rust-next.

[1]: https://lore.kernel.org/all/84bc8bd2-e292-4b84-9580-a1b5df4c5bdc@xxxxxxxxxx/
[2]: https://lore.kernel.org/all/20260710-chid-maple-v1-1-4ee869055268@xxxxxxxxxx/
[3]: https://lore.kernel.org/all/20260717053241.916441-1-ynorov@xxxxxxxxxx/

Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
---
Changes in v2:
- Collected Alice's Reviewed-by on patch 1.
- Address Yury's comments w.r.t. using __bitmap_set etc directly.
- Address Yury's comments w.r.t. following the C names
- Additionally check for an overflow case that causes a hang
- Added more info to cover letter + patch 4 w.r.t. channel ID allottment
  requirements
- Add align parameter to ChannelIdPool::alloc_area() plus an aligned
  allocation test
- Add missing INVARIANT comment when constructing UnusedArea
- Link to v1:
  https://patch.msgid.link/20260703-chid-v1-0-84fe8259e46e@xxxxxxxxxx

---
Eliot Courtney (4):
      rust: bitmap: use function-level cfg on kunit test
      rust: bitmap: add contiguous area operations
      rust: id_pool: add contiguous area allocation
      gpu: nova-core: add ChannelIdPool

 drivers/gpu/nova-core/gpu.rs         |   2 +
 drivers/gpu/nova-core/gpu/channel.rs | 182 ++++++++++++++++++++++++++
 rust/kernel/bitmap.rs                | 242 +++++++++++++++++++++++++++++++++--
 rust/kernel/id_pool.rs               |  69 ++++++++++
 4 files changed, 481 insertions(+), 14 deletions(-)
---
base-commit: 71d4e7233f235871b13553e504e591ace6b54373
change-id: 20260608-chid-18fa943c6d6c

Best regards,
--  
Eliot Courtney <ecourtney@xxxxxxxxxx>