Re: [PATCH v6 9/9] rust: sync: atomic: Add Atomic<{usize,isize}>

On Fri, Jul 11, 2025 at 03:45:32PM +0200, Miguel Ojeda wrote:
> On Fri, Jul 11, 2025 at 11:00 AM Benno Lossin <[email protected]> wrote:
> >
> > Do we have a static assert with these cfgs that `isize` has the same
> > size as these?
> >
> > If not, then it would probably make sense to add them now.
> 
> Yeah, according to e.g. Matthew et al., we may end up with 128-bit
> pointers in the kernel fairly soon (e.g. a decade):
> 
>     https://lwn.net/Articles/908026/
> 
> I rescued part of what I wrote in the old `mod assumptions` which I
> never got to send back then -- most of the `static_asserts` are
> redundant now that we define directly the types in the `ffi` crate (I
> mean, we could still assert that `size_of::<c_char>() == 1` and so on,
> but they are essentially a tautology now), so I adapted the comments.
> Please see below (draft).
> 

Thanks Miguel.

Maybe we can do even better: having a type alias mapping to the exact
i{32,64,128} based on kernel configs? Like

(in kernel/lib.rs or ffi.rs)

// Want to buy a better name ;-)
#[cfg(CONFIG_128BIT)]
type isize_mapping = i128;
#[cfg(CONFIG_64BIT)]
type isize_mapping = i64;
#[cfg(not(any(CONFIG_128BIT, CONFIG_64BIT)))]
type isize_mapping = i32;

similar for usize.

Thoughts?

Regards,
Boqun

> Cheers,
> Miguel

> From afd58f3808bd41cfb92bf1acdf2a19081a439ca3 Mon Sep 17 00:00:00 2001
> From: Miguel Ojeda <[email protected]>
> Date: Fri, 11 Jul 2025 15:27:27 +0200
> Subject: [PATCH] rust: ffi: assert sizes and clarify 128-bit situation
> 
> Link: https://lwn.net/Articles/908026/
> Signed-off-by: Miguel Ojeda <[email protected]>
> ---
>  rust/ffi.rs | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/rust/ffi.rs b/rust/ffi.rs
> index d60aad792af4..bbda56c28ca8 100644
> --- a/rust/ffi.rs
> +++ b/rust/ffi.rs
> @@ -38,11 +38,43 @@ macro_rules! alias {
>  
>      // In the kernel, `intptr_t` is defined to be `long` in all platforms, so we can map the type to
>      // `isize`.
> +    //
> +    // It is likely that the assumption that `long` is the size of a CPU register/pointer will stay
> +    // when support for 128-bit architectures is added, thus these will still mapped to `{i,u}size`.
>      c_long = isize;
>      c_ulong = usize;
>  
> +    // Since `long` will likely be 128-bit for 128-bit architectures, `long long` will likely be
> +    // increased. Thus these may happen to be either 64-bit or 128-bit in the future, and thus new
> +    // code should avoid relying on them being 64-bit.
>      c_longlong = i64;
>      c_ulonglong = u64;
>  }
>  
> +// Thus, `long` may be 32-bit, 64-bit or 128-bit.
> +const _: () = {
> +    assert!(
> +        core::mem::size_of::<c_long>()
> +            == if cfg!(CONFIG_128BIT) {
> +                core::mem::size_of::<u128>()
> +            } else if cfg!(CONFIG_64BIT) {
> +                core::mem::size_of::<u64>()
> +            } else {
> +                core::mem::size_of::<u32>()
> +            }
> +    );
> +};
> +
> +// And `long long` may be 64-bit or 128-bit.
> +const _: () = {
> +    assert!(
> +        core::mem::size_of::<c_longlong>()
> +            == if cfg!(CONFIG_64BIT) {
> +                core::mem::size_of::<u64>()
> +            } else {
> +                core::mem::size_of::<u128>()
> +            }
> +    );
> +};
> +
>  pub use core::ffi::c_void;
> 
> base-commit: d7b8f8e20813f0179d8ef519541a3527e7661d3a
> -- 
> 2.50.1
>