[PATCH v3 0/4] rust: Add support for reserving of ranges of IDs

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/[email protected]/
[2]: https://lore.kernel.org/all/[email protected]/
[3]: https://lore.kernel.org/all/[email protected]/

Signed-off-by: Eliot Courtney <[email protected]>
---
Changes in v3:
- Use `Alignment` type in id_pool and bitmap (Alice)
- Remove hang check on the basis that it's extraordinarily rare.
- Link to v2: https://patch.msgid.link/[email protected]

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/[email protected]

---
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 | 180 ++++++++++++++++++++++++++++
 rust/kernel/bitmap.rs                | 219 ++++++++++++++++++++++++++++++++---
 rust/kernel/id_pool.rs               |  69 +++++++++++
 4 files changed, 456 insertions(+), 14 deletions(-)
---
base-commit: 6dcbb4b1320fa91fee349462a52bb69135f2e45e
change-id: 20260608-chid-18fa943c6d6c

Best regards,
--  
Eliot Courtney <[email protected]>