linux-kernel - [PATCH V3 3/3] rust: cpu: Add CpuId::current() to retrieve current CPU ID

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Message-Id: <a2e12436661fca452de5c417242328ed6f413511.1749554685.git.viresh.kumar@linaro.org>
Date: Tue, 10 Jun 2025 18:51:58 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Boqun Feng <boqun.feng@...il.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Cc: Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Yury Norov <yury.norov@...il.com>,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH V3 3/3] rust: cpu: Add CpuId::current() to retrieve current CPU ID

Introduce `CpuId::current()`, a constructor that wraps the C function
`raw_smp_processor_id()` to retrieve the current CPU identifier without
guaranteeing stability.

This function should be used only when the caller can ensure that
the CPU ID won't change unexpectedly due to preemption or migration.

Suggested-by: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
 MAINTAINERS            |  1 +
 rust/helpers/cpu.c     |  8 ++++++++
 rust/helpers/helpers.c |  1 +
 rust/kernel/cpu.rs     | 10 ++++++++++
 4 files changed, 20 insertions(+)
 create mode 100644 rust/helpers/cpu.c

diff --git a/MAINTAINERS b/MAINTAINERS
index a92290fffa16..4255186784c4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6254,6 +6254,7 @@ F:	include/linux/cpuhotplug.h
 F:	include/linux/smpboot.h
 F:	kernel/cpu.c
 F:	kernel/smpboot.*
+F:	rust/helper/cpu.c
 F:	rust/kernel/cpu.rs
 
 CPU IDLE TIME MANAGEMENT FRAMEWORK
diff --git a/rust/helpers/cpu.c b/rust/helpers/cpu.c
new file mode 100644
index 000000000000..824e0adb19d4
--- /dev/null
+++ b/rust/helpers/cpu.c
@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/smp.h>
+
+unsigned int rust_helper_raw_smp_processor_id(void)
+{
+	return raw_smp_processor_id();
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 0f1b5d115985..16fa9bca5949 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -13,6 +13,7 @@
 #include "build_assert.c"
 #include "build_bug.c"
 #include "clk.c"
+#include "cpu.c"
 #include "cpufreq.c"
 #include "cpumask.c"
 #include "cred.c"
diff --git a/rust/kernel/cpu.rs b/rust/kernel/cpu.rs
index 7549594fad7f..a946c8a9d1a2 100644
--- a/rust/kernel/cpu.rs
+++ b/rust/kernel/cpu.rs
@@ -102,6 +102,16 @@ pub fn from_u32(id: u32) -> Option<Self> {
     pub fn as_u32(&self) -> u32 {
         self.0
     }
+
+    /// Returns the ID of the CPU the code is currently running on.
+    ///
+    /// The returned value is considered unstable because it may change
+    /// unexpectedly due to preemption or CPU migration. It should only be
+    /// used when the context ensures that the task remains on the same CPU.
+    pub fn current() -> Self {
+        // SAFETY: raw_smp_processor_id() always returns a valid CPU ID.
+        unsafe { Self::from_u32_unchecked(bindings::raw_smp_processor_id()) }
+    }
 }
 
 impl From<CpuId> for u32 {
-- 
2.31.1.272.g89b43f80a514