Re: [PATCH v5 13/26] rust: alloc: implement kernel `Vec` type

[Alice Ryhl]

On Thu, Aug 15, 2024 at 12:46 AM Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>
> On Wed, Aug 14, 2024 at 10:42:28AM +0200, Alice Ryhl wrote:
> > > +#[macro_export]
> > > +macro_rules! kvec {
> > > +    () => (
> > > +        {
> > > +            $crate::alloc::KVec::new()
> > > +        }
> > > +    );
> > > +    ($elem:expr; $n:expr) => (
> > > +        {
> > > +            $crate::alloc::KVec::from_elem($elem, $n, GFP_KERNEL)
> > > +        }
> > > +    );
> > > +    ($($x:expr),+ $(,)?) => (
> > > +        {
> > > +            match $crate::alloc::KBox::new([$($x),+], GFP_KERNEL) {
> > > +                Ok(b) => Ok($crate::alloc::KBox::into_vec(b)),
> > > +                Err(e) => Err(e),
> >
> > Hmm. This currently generates code that:
> >
> > 1. Creates the array.
> > 2. Allocates the memory.
> > 3. Moves the array into the box.
> >
> > Whereas the stdlib macro swaps step 1 and 2.
>
> Isn't stdlib [1] doing the same thing I do?
>
> [1] https://doc.rust-lang.org/1.80.1/src/alloc/macros.rs.html#49

Stdlib is using something called #[rustc_box] which has the effect I described.

> > You can do the same by utilizing new_uninit. A sketch:
> >
> > match KBox::<[_; _]>::new_uninit(GFP_KERNEL) {
>
> How do we get the size here? `#![feature(generic_arg_infer)]` seems to be
> unstable.

It probably works if you don't specify the type at all:
`KBox::new_uninit`. But you should double check.

> >     Ok(b) => Ok(KVec::from(KBox::write(b, [$($x),+]))),
> >     Err(e) => Err(e),
> > }