[PATCH v4 0/5] rust: Add support for reserving of ranges of IDs

[PATCH v4 0/5] 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@nvidia.com/
[2]: https://lore.kernel.org/all/20260710-chid-maple-v1-1-4ee869055268@nvidia.com/
[3]: https://lore.kernel.org/all/20260717053241.916441-1-ynorov@nvidia.com/

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
Changes in v4:
- Add `next_zero_area_off` to match C code (Yury)
- Replace overflow checks to match C code in bitmap-for-next.
- Tighten `Bitmap` unsafe contract to disallow Bitmaps larger than i32::MAX
- Link to v3: https://patch.msgid.link/20260729-chid-v3-0-20cc08032bbc@nvidia.com

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/20260723-chid-v2-0-c35e5e9fb3d9@nvidia.com

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@nvidia.com

---
Eliot Courtney (5):
      rust: bitmap: use function-level cfg on kunit test
      rust: bitmap: restrict bitmap length to at most i32::MAX
      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                | 268 +++++++++++++++++++++++++++++++++--
 rust/kernel/id_pool.rs               |  69 +++++++++
 4 files changed, 504 insertions(+), 15 deletions(-)
---
base-commit: 4c9ba407018e8deb06dbc643112bac8f40404f95
change-id: 20260608-chid-18fa943c6d6c

Best regards,
--  
Eliot Courtney <ecourtney@nvidia.com>

[PATCH v4 1/5] rust: bitmap: use function-level cfg on kunit test

[Eliot Courtney]

Since commit c652dc44192d ("rust: kunit: allow `cfg` on `test`s"),
we no longer need this workaround.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 rust/kernel/bitmap.rs | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index b27e0ec80d64..a43bfe0ec3dc 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -572,24 +572,21 @@ fn bitmap_set_clear_find() -> Result<(), AllocError> {
     }
 
     #[test]
+    #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
     fn owned_bitmap_out_of_bounds() -> Result<(), AllocError> {
-        // TODO: Kunit #[test]s do not support `cfg` yet,
-        // so we add it here in the body.
-        #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
-        {
-            let mut b = BitmapVec::new(128, GFP_KERNEL)?;
-            b.set_bit(2048);
-            b.set_bit_atomic(2048);
-            b.clear_bit(2048);
-            b.clear_bit_atomic(2048);
-            assert_eq!(None, b.next_bit(2048));
-            assert_eq!(None, b.next_zero_bit(2048));
-            assert_eq!(None, b.last_bit());
-        }
+        let mut b = BitmapVec::new(128, GFP_KERNEL)?;
+
+        b.set_bit(2048);
+        b.set_bit_atomic(2048);
+        b.clear_bit(2048);
+        b.clear_bit_atomic(2048);
+        assert_eq!(None, b.next_bit(2048));
+        assert_eq!(None, b.next_zero_bit(2048));
+        assert_eq!(None, b.last_bit());
         Ok(())
     }
 
-    // TODO: uncomment once kunit supports [should_panic] and `cfg`.
+    // TODO: uncomment once kunit supports `#[should_panic]`.
     // #[cfg(CONFIG_RUST_BITMAP_HARDENED)]
     // #[test]
     // #[should_panic]

-- 
2.55.0

[PATCH v4 2/5] rust: bitmap: restrict bitmap length to at most i32::MAX

[Eliot Courtney]

It is currently possible to construct a non-`BitmapVec` backed
`Bitmap` using `Bitmap::from_raw` that is larger than `i32::MAX`, and
it is not part of the unsafe requirements. Restricting all bitmaps
(even non-`BitmapVec` backed ones) to a maximum size of `i32::MAX`
simplifies a few things and matches `BitmapVec::MAX_LEN`.

Add that requirement to the unsafe requirements on `Bitmap::from_raw`
and `Bitmap::from_raw_mut`, and to the invariants on `Bitmap`.

This also fixes u32 casts truncating in `copy_and_extend`, which could
otherwise lead to OOB writes.

Fixes: 11eca92a2cae ("rust: add bitmap API.")
Link: https://lore.kernel.org/DKG0U8RLO7LZ.2I1AIH0S38PAP@nvidia.com
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 rust/kernel/bitmap.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index a43bfe0ec3dc..0d481d761f2a 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -17,6 +17,7 @@
 /// # Invariants
 ///
 /// Must reference a `[c_ulong]` long enough to fit `data.len()` bits.
+/// Must not be longer than `i32::MAX` bits.
 #[cfg_attr(CONFIG_64BIT, repr(align(8)))]
 #[cfg_attr(not(CONFIG_64BIT), repr(align(4)))]
 pub struct Bitmap {
@@ -30,11 +31,13 @@ impl Bitmap {
     ///
     /// * `ptr` holds a non-null address of an initialized array of `unsigned long`
     ///   that is large enough to hold `nbits` bits.
+    /// * `nbits` must not exceed `i32::MAX`.
     /// * the array must not be freed for the lifetime of this [`Bitmap`]
     /// * concurrent access only happens through atomic operations
     pub unsafe fn from_raw<'a>(ptr: *const usize, nbits: usize) -> &'a Bitmap {
         let data: *const [()] = core::ptr::slice_from_raw_parts(ptr.cast(), nbits);
         // INVARIANT: `data` references an initialized array that can hold `nbits` bits.
+        // INVARIANT: the caller guarantees that `nbits` does not exceed `i32::MAX`.
         // SAFETY:
         // The caller guarantees that `data` (derived from `ptr` and `nbits`)
         // points to a valid, initialized, and appropriately sized memory region
@@ -55,11 +58,13 @@ pub unsafe fn from_raw<'a>(ptr: *const usize, nbits: usize) -> &'a Bitmap {
     ///
     /// * `ptr` holds a non-null address of an initialized array of `unsigned long`
     ///   that is large enough to hold `nbits` bits.
+    /// * `nbits` must not exceed `i32::MAX`.
     /// * the array must not be freed for the lifetime of this [`Bitmap`]
     /// * no concurrent access may happen.
     pub unsafe fn from_raw_mut<'a>(ptr: *mut usize, nbits: usize) -> &'a mut Bitmap {
         let data: *mut [()] = core::ptr::slice_from_raw_parts_mut(ptr.cast(), nbits);
         // INVARIANT: `data` references an initialized array that can hold `nbits` bits.
+        // INVARIANT: the caller guarantees that `nbits` does not exceed `i32::MAX`.
         // SAFETY:
         // The caller guarantees that `data` (derived from `ptr` and `nbits`)
         // points to a valid, initialized, and appropriately sized memory region
@@ -415,7 +420,8 @@ pub fn clear_bit_atomic(&self, index: usize) {
     #[inline]
     pub fn copy_and_extend(&mut self, src: &Bitmap) {
         let len = core::cmp::min(src.len(), self.len());
-        // SAFETY: access to `self` and `src` is within bounds.
+        // SAFETY: access to `self` and `src` is within bounds. Both lengths fit in `u32`
+        // because a `Bitmap` is at most `i32::MAX` bits, so the casts are lossless.
         unsafe {
             bindings::bitmap_copy_and_extend(
                 self.as_mut_ptr(),

-- 
2.55.0

[PATCH v4 3/5] rust: bitmap: add contiguous area operations

[Eliot Courtney]

Add bindings for area operations on bitmaps. Each one is
made safe by adding some extra checks compared to the underlying C code
(for example, checking bounds) and with additional checks to catch
likely erroneous usage if `CONFIG_RUST_BITMAP_HARDENED` is on.

Add tests demonstrating the edge cases.

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 rust/kernel/bitmap.rs | 235 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 235 insertions(+)

diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index 0d481d761f2a..a2557e9c5cfe 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -10,6 +10,7 @@
 use crate::bindings;
 #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
 use crate::pr_err;
+use crate::ptr::Alignment;
 use core::ptr::NonNull;
 
 /// Represents a C bitmap. Wraps underlying C bitmap API.
@@ -503,6 +504,138 @@ pub fn next_zero_bit(&self, start: usize) -> Option<usize> {
             Some(index)
         }
     }
+
+    /// Finds a contiguous area of `nbits` zero bits at or after `start`, where the area plus
+    /// `align_offset` is aligned to `align`.
+    ///
+    /// Returns the bit index of the start of the area, or [`None`] if no such area fitting in
+    /// the bitmap exists.
+    ///
+    /// The returned index plus `align_offset` is a multiple of `align`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `start` is out of bounds.
+    #[inline]
+    pub fn next_zero_area_off(
+        &self,
+        start: usize,
+        nbits: usize,
+        align: Alignment,
+        align_offset: usize,
+    ) -> Option<usize> {
+        bitmap_assert!(
+            start < self.len(),
+            "`start` must be < {}, was {}",
+            self.len(),
+            start
+        );
+
+        let nr = u32::try_from(nbits).ok()?;
+
+        // The C alignment and end arithmetic must not overflow, or it can read out of bounds.
+        // Overflow is only possible on 32-bit.
+        let align_mask = align.as_usize() - 1;
+        align_mask.checked_add(self.len())?.checked_add(nbits)?;
+
+        // SAFETY: `bitmap_find_next_zero_area_off` is safe to use with an out of bounds `start`
+        // value and, given the overflow check above, never reads beyond `self.len()` bits.
+        let index = unsafe {
+            bindings::bitmap_find_next_zero_area_off(
+                self.as_ptr().cast_mut(),
+                self.len(),
+                start,
+                nr,
+                align_mask,
+                align_offset,
+            )
+        };
+
+        (index < self.len()).then_some(index)
+    }
+
+    /// Finds a contiguous area of `nbits` zero bits at or after `start`, aligned to `align`.
+    ///
+    /// Returns the bit index of the start of the area, or [`None`] if no such area fitting in
+    /// the bitmap exists.
+    ///
+    /// The returned index is a multiple of `align`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and `start` is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::alloc::{AllocError, flags::GFP_KERNEL};
+    /// use kernel::bitmap::BitmapVec;
+    /// use kernel::ptr::Alignment;
+    ///
+    /// let mut b = BitmapVec::new(64, GFP_KERNEL)?;
+    /// let unaligned = Alignment::new::<1>();
+    ///
+    /// assert_eq!(Some(0), b.next_zero_area(0, 8, unaligned));
+    /// b.set(0, 5);
+    /// assert_eq!(Some(5), b.next_zero_area(0, 8, unaligned));
+    /// assert_eq!(Some(8), b.next_zero_area(0, 8, Alignment::new::<8>()));
+    /// assert_eq!(None, b.next_zero_area(0, 65, unaligned));
+    /// # Ok::<(), AllocError>(())
+    /// ```
+    #[inline]
+    pub fn next_zero_area(&self, start: usize, nbits: usize, align: Alignment) -> Option<usize> {
+        self.next_zero_area_off(start, nbits, align, 0)
+    }
+
+    /// Sets a contiguous area of `nbits` bits starting at `start`.
+    ///
+    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and the area `start..start + nbits` is out of
+    /// bounds, does nothing.
+    ///
+    /// # Panics
+    ///
+    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and the area `start..start + nbits` is out
+    /// of bounds.
+    #[inline]
+    pub fn set(&mut self, start: usize, nbits: usize) {
+        bitmap_assert_return!(
+            start
+                .checked_add(nbits)
+                .is_some_and(|end| end <= self.len()),
+            "Area `start..start + nbits` ({}..{}) must be within bounds {}",
+            start,
+            start.saturating_add(nbits),
+            self.len()
+        );
+        // SAFETY: The area `start..start + nbits` is within bounds and a `Bitmap` is at most
+        // `i32::MAX` bits, so the casts are lossless.
+        unsafe { bindings::__bitmap_set(self.as_mut_ptr(), start as u32, nbits as i32) };
+    }
+
+    /// Clears a contiguous area of `nbits` bits starting at `start`.
+    ///
+    /// If CONFIG_RUST_BITMAP_HARDENED is not enabled and the area `start..start + nbits` is out of
+    /// bounds, does nothing.
+    ///
+    /// # Panics
+    ///
+    /// Panics if CONFIG_RUST_BITMAP_HARDENED is enabled and the area `start..start + nbits` is out
+    /// of bounds.
+    #[inline]
+    pub fn clear(&mut self, start: usize, nbits: usize) {
+        bitmap_assert_return!(
+            start
+                .checked_add(nbits)
+                .is_some_and(|end| end <= self.len()),
+            "Area `start..start + nbits` ({}..{}) must be within bounds {}",
+            start,
+            start.saturating_add(nbits),
+            self.len()
+        );
+        // SAFETY: The area `start..start + nbits` is within bounds and a `Bitmap` is at most
+        // `i32::MAX` bits, so the casts are lossless.
+        unsafe { bindings::__bitmap_clear(self.as_mut_ptr(), start as u32, nbits as i32) };
+    }
 }
 
 #[cfg(CONFIG_RUST_BITMAP_KUNIT_TEST)]
@@ -620,4 +753,106 @@ fn bitmap_copy_and_extend() -> Result<(), AllocError> {
         assert_eq!(Some(17), long_bitmap.last_bit());
         Ok(())
     }
+
+    #[test]
+    fn bitmap_area_set_clear_find() -> Result<(), AllocError> {
+        let mut b = BitmapVec::new(128, GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+
+        assert_eq!(Some(0), b.next_zero_area(0, 5, unaligned));
+        b.set(0, 5); // Now contains {[0, 5)}.
+
+        assert_eq!(Some(0), b.next_bit(0));
+        assert_eq!(Some(4), b.next_bit(4));
+        assert_eq!(Some(5), b.next_zero_bit(0));
+        assert_eq!(Some(5), b.next_zero_area(0, 5, unaligned));
+        assert_eq!(Some(8), b.next_zero_area(0, 5, Alignment::new::<8>()));
+
+        b.set(8, 8); // Now contains {[0, 5), [8, 16)}.
+        assert_eq!(Some(16), b.next_zero_area(0, 4, Alignment::new::<16>()));
+        assert_eq!(Some(16), b.next_zero_area(0, 4, unaligned));
+
+        b.clear(0, 5); // Now contains {[8, 16)}.
+        assert_eq!(Some(0), b.next_zero_area(0, 5, unaligned));
+        assert_eq!(Some(8), b.next_bit(0));
+        assert_eq!(Some(15), b.last_bit());
+
+        b.clear(16, 0); // Zero-length in-bounds clears are no-ops.
+        assert_eq!(Some(8), b.next_bit(0));
+        assert_eq!(Some(15), b.last_bit());
+
+        // A zero-length request returns the first aligned position at or
+        // after the next zero bit, even if that position's own bit is set.
+        assert_eq!(Some(1), b.next_zero_area(1, 0, unaligned));
+        assert_eq!(Some(8), b.next_zero_area(1, 0, Alignment::new::<8>()));
+
+        b.set(60, 10); // Now contains {[8, 16), [60, 70)}.
+        assert_eq!(Some(60), b.next_bit(16));
+        assert_eq!(Some(69), b.last_bit());
+        assert_eq!(Some(16), b.next_zero_area(9, 40, unaligned));
+        assert_eq!(Some(70), b.next_zero_area(0, 45, unaligned));
+
+        b.clear(62, 6); // Now contains {[8, 16), [60, 62), [68, 70)}.
+        assert_eq!(Some(62), b.next_zero_area(60, 6, unaligned));
+        assert_eq!(Some(61), b.next_bit(61));
+        assert_eq!(Some(69), b.last_bit());
+
+        b.set(64, 0); // Zero-length in-bounds sets are no-ops.
+        assert_eq!(Some(62), b.next_zero_bit(62));
+        Ok(())
+    }
+
+    #[test]
+    fn bitmap_area_exhaustion() -> Result<(), AllocError> {
+        let mut b = BitmapVec::new(64, GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+
+        assert_eq!(None, b.next_zero_area(0, 65, unaligned));
+        assert_eq!(None, b.next_zero_area(0, usize::MAX, unaligned));
+        assert_eq!(None, b.next_zero_area(1, usize::MAX, unaligned));
+
+        b.set_bit(0); // Now contains {[0, 1)}.
+        assert_eq!(None, b.next_zero_area(0, usize::MAX, unaligned));
+
+        b.set(0, 61); // Now contains {[0, 61)}.
+        assert_eq!(None, b.next_zero_area(0, 4, unaligned));
+        assert_eq!(Some(61), b.next_zero_area(0, 3, unaligned));
+        assert_eq!(None, b.next_zero_area(0, 1, Alignment::new::<64>()));
+        Ok(())
+    }
+
+    #[test]
+    fn bitmap_area_off() -> Result<(), AllocError> {
+        let mut b = BitmapVec::new(64, GFP_KERNEL)?;
+        let align8 = Alignment::new::<8>();
+
+        b.set(0, 5); // Now contains {[0, 5)}.
+
+        // The area plus align_offset starts at a multiple of the alignment.
+        assert_eq!(Some(7), b.next_zero_area_off(0, 8, align8, 1));
+        assert_eq!(Some(5), b.next_zero_area_off(0, 8, align8, 3));
+
+        // A zero offset behaves like next_zero_area().
+        assert_eq!(
+            b.next_zero_area(0, 8, align8),
+            b.next_zero_area_off(0, 8, align8, 0)
+        );
+        Ok(())
+    }
+
+    #[test]
+    #[cfg(not(CONFIG_RUST_BITMAP_HARDENED))]
+    fn owned_bitmap_area_out_of_bounds() -> Result<(), AllocError> {
+        let mut b = BitmapVec::new(64, GFP_KERNEL)?;
+
+        // Should be ignored since out of bounds.
+        b.set(64, 4);
+        b.set(62, 8);
+        b.set(usize::MAX, 0);
+        b.clear(usize::MAX, 0);
+        b.clear(2048, 8);
+        assert_eq!(None, b.next_bit(0));
+        assert_eq!(None, b.next_zero_area(64, 1, Alignment::new::<1>()));
+        Ok(())
+    }
 }

-- 
2.55.0

[PATCH v4 4/5] rust: id_pool: add contiguous area allocation

[Eliot Courtney]

Add support for contiguous area allocation. Add a new type,
`UnusedArea`, following the same pattern as `UnusedId`.

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 rust/kernel/id_pool.rs | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/rust/kernel/id_pool.rs b/rust/kernel/id_pool.rs
index 384753fe0e44..eb911a0e3217 100644
--- a/rust/kernel/id_pool.rs
+++ b/rust/kernel/id_pool.rs
@@ -4,8 +4,14 @@
 
 //! Rust API for an ID pool backed by a [`BitmapVec`].
 
+use core::{
+    num::NonZero,
+    ops::Range, //
+};
+
 use crate::alloc::{AllocError, Flags};
 use crate::bitmap::BitmapVec;
+use crate::ptr::Alignment;
 
 /// Represents a dynamic ID pool backed by a [`BitmapVec`].
 ///
@@ -240,6 +246,33 @@ pub fn find_unused_id(&mut self, offset: usize) -> Option<UnusedId<'_>> {
     pub fn release_id(&mut self, id: usize) {
         self.map.clear_bit(id);
     }
+
+    /// Finds a contiguous area of `count` unused IDs at or after `offset`.
+    ///
+    /// The start of the returned area is a multiple of `align`.
+    ///
+    /// Returns an [`UnusedArea`] upon success, or [`None`] if no such area could be found.
+    #[inline]
+    #[must_use]
+    pub fn find_unused_area(
+        &mut self,
+        offset: usize,
+        count: NonZero<usize>,
+        align: Alignment,
+    ) -> Option<UnusedArea<'_>> {
+        let start = self.map.next_zero_area(offset, count.get(), align)?;
+        // INVARIANT: `next_zero_area()` returns None or a start with `start + count <= map.len()`.
+        Some(UnusedArea {
+            range: start..start + count.get(),
+            pool: self,
+        })
+    }
+
+    /// Releases a contiguous area of IDs.
+    #[inline]
+    pub fn release_area(&mut self, range: &Range<usize>) {
+        self.map.clear(range.start, range.len());
+    }
 }
 
 /// Represents an unused id in an [`IdPool`].
@@ -287,6 +320,42 @@ pub fn acquire(self) -> usize {
     }
 }
 
+/// Represents an unused, contiguous area of IDs in an [`IdPool`].
+///
+/// # Invariants
+///
+/// `range.start <= range.end <= pool.map.len()`.
+#[must_use = "the ID range is not reserved unless acquired"]
+pub struct UnusedArea<'pool> {
+    range: Range<usize>,
+    pool: &'pool mut IdPool,
+}
+
+impl<'pool> UnusedArea<'pool> {
+    /// Returns the unused ID range.
+    ///
+    /// Be aware that the area has not yet been acquired in the pool. The
+    /// [`acquire`] method must be called to prevent others from taking it.
+    ///
+    /// [`acquire`]: UnusedArea::acquire()
+    #[inline]
+    #[must_use]
+    pub fn range(&self) -> Range<usize> {
+        self.range.clone()
+    }
+
+    /// Acquires the area.
+    ///
+    /// Returns the now-reserved ID range.
+    #[inline]
+    pub fn acquire(self) -> Range<usize> {
+        let Self { range, pool } = self;
+        // By the type invariants, the range is within bounds.
+        pool.map.set(range.start, range.end - range.start);
+        range
+    }
+}
+
 impl Default for IdPool {
     #[inline]
     fn default() -> Self {

-- 
2.55.0

[PATCH v4 5/5] gpu: nova-core: add ChannelIdPool

[Eliot Courtney]

Add `ChannelIdPool` which adds automatic tracking and releasing of
channel IDs on top of `IdPool`. This is necessary for apportioning
ranges of channel IDs to be used in e.g. vGPU.

Channel IDs are allocated as a contiguous sequence with a specific
length and sometimes a specific alignment [1] for vGPU. The ID space is
small (limited to 2048) and allocation is not on a hot path, so a
bitmap-backed `IdPool` is a better fit than IDA/xarray (which allocate a
single ID within a range, not a contiguous sequence) or a maple tree
(where aligned allocation needs an alloc_range()+erase() retry loop that
essentially reimplements bitmap_find_next_zero_area()) [2]. It is
also faster than maple tree [3].

Link: https://lore.kernel.org/all/84bc8bd2-e292-4b84-9580-a1b5df4c5bdc@nvidia.com/ # [1]
Link: https://lore.kernel.org/all/20260710-chid-maple-v1-1-4ee869055268@nvidia.com/ # [2]
Link: https://lore.kernel.org/all/20260717053241.916441-1-ynorov@nvidia.com/ # [3]
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 drivers/gpu/nova-core/gpu.rs         |   2 +
 drivers/gpu/nova-core/gpu/channel.rs | 180 +++++++++++++++++++++++++++++++++++
 2 files changed, 182 insertions(+)

diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index 42a4cd7971fa..66ea697a89f8 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -33,6 +33,8 @@
     vgpu::VgpuManager, //
 };
 
+#[cfg_attr(not(CONFIG_KUNIT = "y"), expect(dead_code))]
+mod channel;
 mod hal;
 
 macro_rules! define_chipset {
diff --git a/drivers/gpu/nova-core/gpu/channel.rs b/drivers/gpu/nova-core/gpu/channel.rs
new file mode 100644
index 000000000000..b755d2184aee
--- /dev/null
+++ b/drivers/gpu/nova-core/gpu/channel.rs
@@ -0,0 +1,180 @@
+// SPDX-License-Identifier: GPL-2.0
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+
+//! Channel ID allocation.
+
+use core::{
+    num::NonZero,
+    ops::{
+        Deref,
+        Range, //
+    }, //
+};
+
+use kernel::{
+    id_pool::IdPool,
+    prelude::*,
+    ptr::Alignment,
+    sync::{
+        new_mutex,
+        Mutex, //
+    }, //
+};
+
+/// Pool for tracking reservations of channel IDs.
+#[pin_data]
+pub(crate) struct ChannelIdPool {
+    #[pin]
+    inner: Mutex<IdPool>,
+    num_chids: usize,
+}
+
+impl ChannelIdPool {
+    /// Creates a pool managing `num_chids` channel IDs.
+    pub(crate) fn new(num_chids: usize) -> impl PinInit<Self, Error> {
+        try_pin_init!(Self {
+            inner <- new_mutex!(IdPool::with_capacity(num_chids, GFP_KERNEL)?),
+            num_chids,
+        })
+    }
+
+    /// Reserves a contiguous area of `count` channel IDs starting at a multiple of `align`,
+    /// returning a guard that releases the area on drop.
+    pub(crate) fn alloc_area(
+        &self,
+        count: NonZero<usize>,
+        align: Alignment,
+    ) -> Result<ChannelIdArea<'_>> {
+        let mut ids = self.inner.lock();
+        let area = ids.find_unused_area(0, count, align).ok_or(ENOSPC)?;
+
+        // If the pool is small, the backing bitmap may be rounded up to a larger size.
+        if area.range().end > self.num_chids {
+            return Err(ENOSPC);
+        }
+        Ok(ChannelIdArea {
+            pool: self,
+            range: area.acquire(),
+        })
+    }
+}
+
+/// A reserved contiguous area of channel IDs.
+///
+/// Releases the whole area back to its [`ChannelIdPool`] when dropped. Releasing locks a
+/// sleeping [`Mutex`], so the area must be dropped in a context that is allowed to sleep.
+#[must_use = "the channel ID area is released immediately when unused"]
+pub(crate) struct ChannelIdArea<'a> {
+    pool: &'a ChannelIdPool,
+    range: Range<usize>,
+}
+
+impl Drop for ChannelIdArea<'_> {
+    fn drop(&mut self) {
+        self.pool.inner.lock().release_area(&self.range);
+    }
+}
+
+impl Deref for ChannelIdArea<'_> {
+    type Target = Range<usize>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.range
+    }
+}
+
+#[kunit_tests(nova_core_channel)]
+mod tests {
+    use super::*;
+
+    const fn nz<const N: usize>() -> NonZero<usize> {
+        const { NonZero::new(N).unwrap() }
+    }
+
+    #[test]
+    fn chid_area() -> Result {
+        let pool = KBox::pin_init(ChannelIdPool::new(2048), GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+
+        let first = pool.alloc_area(nz::<48>(), unaligned)?;
+        assert_eq!(0, first.start);
+        assert_eq!(48, first.len());
+        assert_eq!(48, first.end);
+
+        let second = pool.alloc_area(nz::<48>(), unaligned)?;
+        assert!(first.end <= second.start || second.end <= first.start);
+
+        let first_start = first.start;
+        drop(first);
+        assert_eq!(first_start, pool.alloc_area(nz::<48>(), unaligned)?.start);
+        Ok(())
+    }
+
+    #[test]
+    fn chid_bounded_by_num_chids() -> Result {
+        let pool = KBox::pin_init(ChannelIdPool::new(4), GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+
+        {
+            let a = pool.alloc_area(nz::<1>(), unaligned)?;
+            let b = pool.alloc_area(nz::<1>(), unaligned)?;
+            let c = pool.alloc_area(nz::<1>(), unaligned)?;
+            let d = pool.alloc_area(nz::<1>(), unaligned)?;
+            assert_eq!(0, a.start);
+            assert_eq!(1, b.start);
+            assert_eq!(2, c.start);
+            assert_eq!(3, d.start);
+            assert_eq!(
+                Err(ENOSPC),
+                pool.alloc_area(nz::<1>(), unaligned).map(|_| ())
+            );
+        }
+
+        assert_eq!(0, pool.alloc_area(nz::<4>(), unaligned)?.start);
+        assert_eq!(
+            Err(ENOSPC),
+            pool.alloc_area(nz::<5>(), unaligned).map(|_| ())
+        );
+
+        let head = pool.alloc_area(nz::<3>(), unaligned)?;
+        assert_eq!(0, head.start);
+        assert_eq!(
+            Err(ENOSPC),
+            pool.alloc_area(nz::<2>(), unaligned).map(|_| ())
+        );
+        assert_eq!(3, pool.alloc_area(nz::<1>(), unaligned)?.start);
+        Ok(())
+    }
+
+    #[test]
+    fn chid_area_aligned() -> Result {
+        let pool = KBox::pin_init(ChannelIdPool::new(16), GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+        let align4 = Alignment::new::<4>();
+
+        // Alloc 0 so the first fit for the next area is unaligned.
+        let pad = pool.alloc_area(nz::<1>(), unaligned)?;
+        assert_eq!(0, pad.start);
+
+        let a = pool.alloc_area(nz::<4>(), align4)?;
+        assert_eq!(4, a.start);
+
+        // The area skipped over by the aligned allocation should still be available.
+        let b = pool.alloc_area(nz::<1>(), unaligned)?;
+        assert_eq!(1, b.start);
+
+        let c = pool.alloc_area(nz::<8>(), Alignment::new::<8>())?;
+        assert_eq!(8, c.start);
+
+        // Only 2 IDs left.
+        assert_eq!(Err(ENOSPC), pool.alloc_area(nz::<4>(), align4).map(|_| ()));
+        assert_eq!(
+            Err(ENOSPC),
+            pool.alloc_area(nz::<1>(), Alignment::new::<32>())
+                .map(|_| ())
+        );
+
+        assert_eq!(2, pool.alloc_area(nz::<2>(), unaligned)?.start);
+        Ok(())
+    }
+}

-- 
2.55.0