Re: [PATCH v4] rust: add a wrapper for the `num_possible_cpus` C function

[Viresh Kumar]

On 30-10-26, 17:06, Andreas Hindborg wrote:
> This function returns the maximum number of CPUs that can be online.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
> Assisted-by: LLM
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> ---
> The Rust null block driver needs this wrapper to put an upper bound on
> the number of submit and poll queues it sets up: it never needs more
> queues than the number of CPUs that can come online.
> 
> This patch was part of the `rust: add a few helpers` series up to v3.
> It is independent of the other two patches in that series, so it is
> sent on its own from v4 on.
> 
> Changes in v4:
> - Move the C helper into `rust/helpers/cpumask.c` and the Rust wrapper
>   into `kernel::cpumask`, to mirror the C side (Miguel, Yury).
> - Put `__rust_helper` on a line of its own for greppability (Yury).
> - Document that the return value counts possible CPUs and does not
>   bound CPU IDs (Peter).
> - Split this patch out of the `rust: add a few helpers` series, as it
>   is independent of the remaining patches (Miguel).
> - Rebase on v7.3.
> - Link to v3: https://msgid.link/20260929-cpu-helpers-v3-0-e968ce5ee613@xxxxxxxxxx
> ---
>  rust/helpers/cpumask.c |  6 ++++++
>  rust/kernel/cpumask.rs | 13 +++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/rust/helpers/cpumask.c b/rust/helpers/cpumask.c
> index 5deced5b975e..43e9d40e0460 100644
> --- a/rust/helpers/cpumask.c
> +++ b/rust/helpers/cpumask.c
> @@ -81,3 +81,9 @@ void rust_helper_free_cpumask_var(cpumask_var_t mask)
>  	free_cpumask_var(mask);
>  }
>  #endif
> +
> +__rust_helper
> +unsigned int rust_helper_num_possible_cpus(void)
> +{
> +	return num_possible_cpus();
> +}
> diff --git a/rust/kernel/cpumask.rs b/rust/kernel/cpumask.rs
> index 44bb36636ee3..67664ec8fbd7 100644
> --- a/rust/kernel/cpumask.rs
> +++ b/rust/kernel/cpumask.rs
> @@ -16,6 +16,19 @@
>  
>  use core::ops::{Deref, DerefMut};
>  
> +/// Returns the maximum number of CPUs that may be online on the system.
> +///
> +/// This is a count of CPUs, not a bound on CPU IDs. Possible CPUs need not occupy a contiguous
> +/// range of IDs, so a CPU ID must be bounded by [`nr_cpu_ids()`] instead. That is,
> +/// `num_possible_cpus() <= nr_cpu_ids()`.
> +///
> +/// [`nr_cpu_ids()`]: crate::cpu::nr_cpu_ids
> +#[inline]
> +pub fn num_possible_cpus() -> u32 {
> +    // SAFETY: FFI call with no additional requirements.
> +    unsafe { bindings::num_possible_cpus() }
> +}

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

-- 
viresh