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@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.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

On Tue, Jun 10, 2025 at 06:51:58PM +0530, Viresh Kumar wrote:
> 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@gmail.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.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.

I would also add "or the users could use a stale (yet valid) CPU ID",
because there are such usages in kernel arleady, e.g.
kvm_pmu_probe_armpmu() in arch/arm64/kvm/pmu-emul.c.

With that, feel free to add:

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> +    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
>

On 10-06-25, 14:23, Boqun Feng wrote:
> I would also add "or the users could use a stale (yet valid) CPU ID",
> because there are such usages in kernel arleady, e.g.
> kvm_pmu_probe_armpmu() in arch/arm64/kvm/pmu-emul.c.

Done.

> With that, feel free to add:
> 
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Thanks Boqun. I have queued this series for linux-next to get some
testing while the reviews are going on. It would be nice if I can get
some reviews on the other patches, especially the one that adds the
`CpuId` abstraction.

-- 
viresh