Add an iterator for `Cpumask` making use of C's `cpumask_next`.

Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>
---
 rust/helpers/cpumask.c |  5 +++++
 rust/kernel/cpumask.rs | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/rust/helpers/cpumask.c b/rust/helpers/cpumask.c
index eb10598a0242..d95bfa111191 100644
--- a/rust/helpers/cpumask.c
+++ b/rust/helpers/cpumask.c
@@ -42,6 +42,11 @@ bool rust_helper_cpumask_full(struct cpumask *srcp)
 	return cpumask_full(srcp);
 }
 
+unsigned int rust_helper_cpumask_next(int n, struct cpumask *srcp)
+{
+	return cpumask_next(n, srcp);
+}
+
 unsigned int rust_helper_cpumask_weight(struct cpumask *srcp)
 {
 	return cpumask_weight(srcp);
diff --git a/rust/kernel/cpumask.rs b/rust/kernel/cpumask.rs
index 3fcbff438670..b7401848f59e 100644
--- a/rust/kernel/cpumask.rs
+++ b/rust/kernel/cpumask.rs
@@ -6,7 +6,7 @@
 
 use crate::{
     alloc::{AllocError, Flags},
-    cpu::CpuId,
+    cpu::{self, CpuId},
     prelude::*,
     types::Opaque,
 };
@@ -161,6 +161,52 @@ pub fn copy(&self, dstp: &mut Self) {
     }
 }
 
+/// Iterator for a `Cpumask`.
+pub struct CpumaskIter<'a> {
+    mask: &'a Cpumask,
+    last: Option<u32>,
+}
+
+impl<'a> CpumaskIter<'a> {
+    /// Creates a new `CpumaskIter` for the given `Cpumask`.
+    fn new(mask: &'a Cpumask) -> CpumaskIter<'a> {
+        Self { mask, last: None }
+    }
+}
+
+impl<'a> Iterator for CpumaskIter<'a> {
+    type Item = CpuId;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // SAFETY: By the type invariant, `self.mask.as_raw` is a `struct cpumask *`.
+        let next = unsafe {
+            bindings::cpumask_next(
+                if let Some(last) = self.last {
+                    last.try_into().unwrap()
+                } else {
+                    -1
+                },
+                self.mask.as_raw(),
+            )
+        };
+
+        if next == cpu::nr_cpu_ids() {
+            None
+        } else {
+            self.last = Some(next);
+            // SAFETY: `cpumask_next` returns either `nr_cpu_ids` or a valid CPU ID.
+            unsafe { Some(CpuId::from_u32_unchecked(next)) }
+        }
+    }
+}
+
+impl Cpumask {
+    /// Returns an iterator over the set bits in the cpumask.
+    pub fn iter(&self) -> CpumaskIter<'_> {
+        CpumaskIter::new(self)
+    }
+}
+
 /// A CPU Mask pointer.
 ///
 /// Rust abstraction for the C `struct cpumask_var_t`.

-- 
2.34.1