Re: [PATCH 4/5] rust: percpu: Add pin-hole optimizations for numerics

["Christoph Lameter (Ampere)" <cl@gentwo.org>]

On Tue, 24 Jun 2025, Mitchell Levy wrote:

> The C implementations of `this_cpu_add`, `this_cpu_sub`, etc., are
> optimized to save an instruction by avoiding having to compute
> `this_cpu_ptr(&x)` for some per-CPU variable `x`. For example, rather
> than

Cool. Great progress for Rust support. Maybe we can switch the SLUB
allocator over or come up with SLRB for the Slab Rust allocator ;-)


> +        impl PerCpuNumeric<'_, $ty> {
> +            /// Adds `rhs` to the per-CPU variable.
> +            pub fn add(&mut self, rhs: $ty) {
> +                // SAFETY: `self.ptr.0` is a valid offset into the per-CPU area (i.e., valid as a
> +                // pointer relative to the `gs` segment register) by the invariants of PerCpu.
> +                unsafe {
> +                    asm!(
> +                        concat!("add gs:[{off}], {val}"),
> +                        off = in(reg) self.ptr.0 as *mut $ty,
> +                        val = in(reg_byte) rhs,

That looks arch specific to x86? What about ARM and other platforms?