Re: [PATCH] rust: add `CacheAligned` for easy cache line alignment of values

[Andreas Hindborg]

"Gary Guo" <gary@xxxxxxxxxxx> writes:

> On Tue May 19, 2026 at 9:18 AM BST, Andreas Hindborg wrote:
>> "Gary Guo" <gary@xxxxxxxxxxx> writes:
>>
>>> On Mon May 18, 2026 at 2:41 PM BST, Andreas Hindborg wrote:
>>>> "Gary Guo" <gary@xxxxxxxxxxx> writes:
>>>>
>>>>> On Wed Jan 28, 2026 at 2:25 PM GMT, Alexandre Courbot wrote:
>>>>>> On Wed Jan 28, 2026 at 11:05 PM JST, Andreas Hindborg wrote:
>>>>>>> `CacheAligned` allows to easily align values to a 64 byte boundary.
>>>>>>>
>>>>>>> An example use case is the kernel `struct spinlock`. This struct is 4 bytes
>>>>>>> on x86 when lockdep is not enabled. The structure is not padded to fit a
>>>>>>> cache line. The effect of this for `SpinLock` is that the lock variable and
>>>>>>> the value protected by the lock might share a cache line, depending on the
>>>>>>> alignment requirements of the protected value. Wrapping the value in
>>>>>>> `CacheAligned` to get a `SpinLock<CacheAligned<T>>` solves this problem.
>>>>>>>
>>>>>>> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
>>>>>>> ---
>>>>>>> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
>>>>>>> ---
>>>>>>>  rust/kernel/cache_aligned.rs | 59 ++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>  rust/kernel/lib.rs           |  2 ++
>>>>>>>  2 files changed, 61 insertions(+)
>>>>>>>
>>>>>>> diff --git a/rust/kernel/cache_aligned.rs b/rust/kernel/cache_aligned.rs
>>>>>>> new file mode 100644
>>>>>>> index 0000000000000..9c33b8613c077
>>>>>>> --- /dev/null
>>>>>>> +++ b/rust/kernel/cache_aligned.rs
>>>>>>> @@ -0,0 +1,59 @@
>>>>>>> +// SPDX-License-Identifier: GPL-2.0
>>>>>>> +
>>>>>>> +use kernel::try_pin_init;
>>>>>>> +use pin_init::{
>>>>>>> +    pin_data,
>>>>>>> +    pin_init,
>>>>>>> +    PinInit, //
>>>>>>> +};
>>>>>>> +
>>>>>>> +/// Wrapper type that alings content to a 64 byte cache line.
>>>>>>
>>>>>> nit: s/alings/aligns
>>>>>>
>>>>>>> +#[repr(align(64))]
>>>>>>
>>>>>> While 64 bytes is the most common cache line size, AFAIK this is not
>>>>>> a universal value? Can we expose and use `L1_CACHE_BYTES` here?
>>>>>
>>>>> Unfortunately `repr(align())` does not accept expression or macro invocations.
>>>>> It's still possible with code-generation, but it'll be more tricky.
>>>>>
>>>>> On all archs that we do support today, I think the value is always 64 However
>>>>> it'd worth putting a FIXME or TODO (or assertion, maybe?) in case new archs gets
>>>>> addded where this isn't true.
>>>>
>>>> I was looking into how to implement this properly. Apparently, we don't
>>>> have a config item that specifies the L1 cache line size for all
>>>> architectures. Each architectures defines the cache line size as C
>>>> define in a header. For x86 we have
>>>>
>>>>   #define L1_CACHE_SHIFT	(CONFIG_X86_L1_CACHE_SHIFT)
>>>>
>>>> and for arm64
>>>>
>>>>   #define L1_CACHE_SHIFT		(6)
>>>>
>>>> and so on.
>>>>
>>>> One thing we could do is run the C preprocessor on a small snippet to
>>>> expand the `L1_CACHE_SHIFT` symbol at some point before invoking
>>>> `rustc`. Then we can pass the value to `rustc` via environment variable
>>>> when building the `macros` crate. This is similar to how we pass
>>>> `RUST_MODFILE` to `rustc`, sans the cpp invocation.
>>>>
>>>> Otherwise we have to convince all architectures that support Rust to
>>>> emit a config that we can rely on, like `CONFIG_L1_CACHE_SHIFT`.
>>>>
>>>> The latter options is probably the better one, what do you all think?
>>>>
>>>> Best regards,
>>>> Andreas Hindborg
>>>
>>> You can implement this with generics alone using associative types.
>>>
>>> mod sealed {
>>>     pub trait Sealed {
>>>         type Repr;
>>>     }
>>> }
>>>
>>> trait Alignment: sealed::Sealed {}
>>>
>>> #[repr(transparent)]
>>> struct Align<const N: usize>([<Self as sealed::Sealed>::Repr; 0])
>>> where
>>>     Self: Alignment;
>>>
>>> impl<const N: usize> Align<N>
>>> where
>>>     Self: Alignment,
>>> {
>>>     #[inline]
>>>     pub fn new() -> Self {
>>>         Align([])
>>>     }
>>> }
>>>
>>> macro_rules! impl_align {
>>>     () => {};
>>>     ($a:literal $($rest:literal)*) => {
>>>         const _: () = {
>>>             #[repr(align($a))]
>>>             struct Repr;
>>>
>>>             impl sealed::Sealed for Align<$a> {
>>>                 type Repr = Repr;
>>>             }
>>>
>>>             impl Alignment for Align<$a> {}
>>>         };
>>>
>>>         impl_align!($($rest)*);
>>>     }
>>> }
>>>
>>> impl_align!(32 64 128 256);
>>
>> Right, that is great for avoiding code duplication in case we want to
>> have `Align<16>`, `Align<32>`, etc.
>>
>> My discussion was about having just `CacheAligned` and having the
>> alignment of that type automatically be the `1 << L1_CACHE_SHIFT`.
>
> #[repr(transparent)]
> struct CacheAligned(Align<{ 1 << L1_CACHE_SHIFT }>);

Oh, I see. I somehow got the idea that this was gated behind
`generic_const_exprs`. But apparently that is only if a generic
parameter is part of the expression 🤦


Best regards,
Andreas Hindborg