Re: [PATCH] rust: page: add volatile memory copy methods

[Andreas Hindborg <a.hindborg@kernel.org>]

"Boqun Feng" <boqun@kernel.org> writes:

> On Sat, Jan 31, 2026 at 08:10:21PM +0100, Andreas Hindborg wrote:
>> "Boqun Feng" <boqun@kernel.org> writes:
>>
>> > On Sat, Jan 31, 2026 at 02:19:05PM +0100, Andreas Hindborg wrote:
>> > [..]
>> >> >
>> >> > However, byte-wise atomic memcpy will be more defined without paying any
>> >> > extra penalty.
>> >>
>> >> Could you explain the additional penalty of `core::ptr::read_volatile`
>> >> vs `kernel::sync::atomic::Atomic::load` with  relaxed ordering?
>> >>
>> >
>> > I don't understand your question, so allow me to explain what I meant:
>> > for the sake of discussion, let's assume we have both
>> >
>> > 	fn volatile_copy_memory(src: *mut u8, dst: *mut u8, count: usize)
>> >
>> > and
>> >
>> > 	fn volatile_byte_wise_atomic_copy_memory(<same signature>, ordering: Ordering)
>> >
>> > implemented. What I meant was to the best of my knowledge, when ordering
>> > = Relaxed, these two would generate the exact same code because all the
>> > architectures that I'm aware of have byte wise atomicity in the
>> > load/store instructions. And compared to volatile_copy_memory(),
>> > volatile_byte_wise_atomic_copy_memory() can bear the race with another
>> > volatile_byte_wise_atomic_copy_memory() or any other atomic access
>> > (meaning that's not a UB). So I'd prefer using that if we have it.
>>
>> Ok, thanks for clarifying. I assumed you were referring to the other
>> functions I mentioned, because they exist in `kernel` or `core`.
>> `volatile_copy_memory` is unstable in `core`, and as far as I know
>> `volatile_byte_wise_atomic_copy_memory` does not exist.
>
> I was using volatile_byte_wise_atomic_copy_memory() to represent the
> concept that we have a volatile byte-wise atomic memcpy. I was trying to
> discuss the performance difference (which is 0) between a "volatile
> memory copy" and "a volatile byte-wise atomic memory copy" based on
> these concepts to answer your question about the "penalty" part of my
> previous reply.
>
>>
>> When you wrote `read_volatile`, I assumed you meant
>> `core::ptr::read_volatile`, and the atomics we have are
>> `kernel::sync::atomic::*`.
>
> It was the curse of knowledge, when I referred to "byte-wise atomic
> memcpy", I meant the concept of this [1], i.e. a memcpy that provides
> atomicity of each byte.
>
> [1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1478r7.html
>
>>
>> So now I am a bit confused as to what method you think is usable here.
>> Is it something we need to implement?
>>
>
> First, since the length of the copy is not fixed, we will need something
> like `volatile_copy_memcpy()` to handle that. So I need to take back my
> previous suggestion about using `read_volatile()`, not because it would
> cause UB, but because it doesn't handle variable lengths.

We could call it in a loop? Would that be inefficient?

>
> But if there could be a concurrent writer to the page we are copying
> from, we need a `volatile_byte_wise_atomic_copy_memory()` that we need
> either implement on our own or ask Rust to provide one.
>
> Does this help?

Yes, this is all super helpful and much appreciated. Thanks!


Best regards,
Andreas Hindborg