linux-kernel - Re: [PATCH v3 4/7] rust: cpumask: Add getters for globally defined cpumasks

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Message-ID: <68b9eec3.170a0220.67a17.2278@mx.google.com>
Date: Thu, 4 Sep 2025 12:55:44 -0700
From: Mitchell Levy <levymitchell0@...il.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Benno Lossin <lossin@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Tyler Hicks <code@...icks.com>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 4/7] rust: cpumask: Add getters for globally defined
 cpumasks

On Wed, Sep 03, 2025 at 06:03:51PM -0400, Yury Norov wrote:
> On Thu, Aug 28, 2025 at 12:00:11PM -0700, 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
> > 
> > Signed-off-by: Mitchell Levy <levymitchell0@...il.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..ca9aa6875b4a 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() -> &'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) }
> > +    }
> 
> This function and the following look like returning a boolean.
> Maybe possible_cpus()? 

Agreed, I think that's definitely better. Will do for v4.

> The general rule is to keep the rust naming as close to C as
> possible, but in this case Cpumask::cpu_possible_mask is, OK,
> somewhat excessive.

Agreed :)

Thanks,
Mitchell

> Thanks,
> Yury
> 
> > +
> > +    /// Get a CPU mask representing online CPUs; has bit `cpu` set iff cpu available to the
> > +    /// scheduler
> > +    #[inline]
> > +    pub fn online() -> &'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() -> &'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() -> &'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() -> &'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