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

[Danilo Krummrich <dakr@kernel.org>]

On 8/15/24 9:30 AM, Alice Ryhl wrote:
> On Thu, Aug 15, 2024 at 12:46 AM Danilo Krummrich <dakr@kernel.org> 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.

That works, thanks.

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