Re: [PATCH v4 2/9] rust: cpumask: Add getters for globally defined cpumasks

[Mitchell Levy <levymitchell0@gmail.com>]

On Thu, Nov 06, 2025 at 07:53:22PM -0500, Yury Norov wrote:
> On Wed, Nov 05, 2025 at 03:01:14PM -0800, Mitchell Levy wrote:
> > Add getters for the global cpumasks documented in
> > `include/linux/cpumask.h`, specifically:
> > - cpu_possible_mask
> > - cpu_online_mask
> > - cpu_enabled_mask
> > - cpu_present_mask
> > - cpu_active_mask
> > 
> > Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> > Suggested-by: Yury Norov <yury.norov@gmail.com>
> > Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>
> > ---
> >  rust/kernel/cpumask.rs | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 46 insertions(+)
> > 
> > diff --git a/rust/kernel/cpumask.rs b/rust/kernel/cpumask.rs
> > index b7401848f59e..a6a130092fcb 100644
> > --- a/rust/kernel/cpumask.rs
> > +++ b/rust/kernel/cpumask.rs
> > @@ -77,6 +77,52 @@ pub unsafe fn as_ref<'a>(ptr: *const bindings::cpumask) -> &'a Self {
> >          unsafe { &*ptr.cast() }
> >      }
> >  
> > +    /// Get a CPU mask representing possible CPUs; has bit `cpu` set iff cpu is populatable
> > +    #[inline]
> > +    pub fn possible_cpus() -> &'static Self {
> > +        // SAFETY: `__cpu_possible_mask` is a valid global provided by the kernel that lives
> > +        // forever.
> > +        unsafe { Cpumask::as_ref(&raw const bindings::__cpu_possible_mask) }
> > +    }
> 
> Not sure about this '&raw const' syntax, but I want to make sure that
> all this getters provide non-modifiable references. To modify any of
> those masks, one has to call a dedicated helper like set_cpu_possible().

For `id: T`, the syntax `&raw const id` produces a `*const T` pointing
at `id` (the analogous syntax `&raw mut id` produces a `*mut T`).

All getters here provide const references (`&'static Self` as opposed to
`&'static mut Self`) which will cause the compiler to prevent usage of
any `Cpumask` methods with a `&mut self` reciever (i.e., methods that
change the `Cpumask`).

> Can you maybe explicitly mention it in the comments?

The compiler will prevent using these values to change the underlying
`__cpu_possible_mask` etc, so mentioning that these return values can't
be used for that purpose seems redundant to me.

It is possible someone could get the pointer out via `Cpumask::as_raw`,
and then do something bad with that pointer, but this isn't any
different than someone doing something bad with
`bindings::__cpu_possible_mask`.

Thanks,
Mitchell

> With that,
> 
> Acked-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> 
> > +    /// Get a CPU mask representing online CPUs; has bit `cpu` set iff cpu available to the
> > +    /// scheduler
> > +    #[inline]
> > +    pub fn online_cpus() -> &'static Self {
> > +        // SAFETY: `__cpu_online_mask` is a valid global provided by the kernel that lives forever.
> > +        // Since we wrap the returned pointer in an `Opaque`, it's ok that `__cpu_online_mask`
> > +        // may change its value.
> > +        unsafe { Cpumask::as_ref(&raw const bindings::__cpu_online_mask) }
> > +    }
> > +
> > +    /// Get a CPU mask representing enabled CPUs; has bit `cpu` set iff cpu can be brought online
> > +    #[inline]
> > +    pub fn enabled_cpus() -> &'static Self {
> > +        // SAFETY: `__cpu_enabled_mask` is a valid global provided by the kernel that lives forever.
> > +        // Since we wrap the returned pointer in an `Opaque`, it's ok that `__cpu_enabled_mask`
> > +        // may change its value.
> > +        unsafe { Cpumask::as_ref(&raw const bindings::__cpu_enabled_mask) }
> > +    }
> > +
> > +    /// Get a CPU mask representing present CPUs; has bit `cpu` set iff cpu is populated
> > +    #[inline]
> > +    pub fn present_cpus() -> &'static Self {
> > +        // SAFETY: `__cpu_present_mask` is a valid global provided by the kernel that lives
> > +        // forever. Since we wrap the returned pointer in an `Opaque`, it's ok that
> > +        // `__cpu_present_mask` may change its value.
> > +        unsafe { Cpumask::as_ref(&raw const bindings::__cpu_present_mask) }
> > +    }
> > +
> > +    /// Get a CPU mask representing active CPUs; has bit `cpu` set iff cpu is available to
> > +    /// migration.
> > +    #[inline]
> > +    pub fn active_cpus() -> &'static Self {
> > +        // SAFETY: `__cpu_active_mask` is a valid global provided by the kernel that lives forever.
> > +        // Since we wrap the returned pointer in an `Opaque`, it's ok that `__cpu_active_mask`
> > +        // may change its value.
> > +        unsafe { Cpumask::as_ref(&raw const bindings::__cpu_active_mask) }
> > +    }
> > +
> >      /// Obtain the raw `struct cpumask` pointer.
> >      pub fn as_raw(&self) -> *mut bindings::cpumask {
> >          let this: *const Self = self;
> > 
> > -- 
> > 2.34.1