Re: [PATCH v3] rust: page: add byte-wise atomic memory copy methods

[Peter Zijlstra]

On Wed, Feb 18, 2026 at 11:36:20AM +0000, Gary Guo wrote:
> On 2026-02-18 10:20, Peter Zijlstra wrote:
> > On Tue, Feb 17, 2026 at 11:10:15PM +0000, Gary Guo wrote:
> > 
> >> If we have this in stable, I think it's sufficient for LKMM. However
> >> for Rust/C11 MM says that volatile ops are not atomic and use them for
> >> concurrency is UB.
> >> 
> >> I recall in last Rust all hands the vibe at discussion is that it's
> >> desirable to define volatile as being byte-wise atomic, so if that
> >> actually happens, this would indeed be what we want (but I think
> >> semantics w.r.t. mixed-size atomics need to be figured out first).
> > 
> > I would strongly suggest for volatile to be single-copy 'atomic' for any
> > naturally aligned word sized access. This is what we have with
> > GCC/Clang.
> > 
> > If you pick anything else, you're explicitly creation interoperability
> > issues.
> 
> AFAIK LLVM IR only "guarantees" this for primitives, so if you have a struct
> that happens to be word-aligned and word-sized, it can still tear, which is
> why the the "byte-wise atomicity" semantics is what's being proposed.

Urgh, what does GCC do? And are we sure this doesn't actually break
anything? I'm fairly sure we rely on at least 'small' struct volatile
reads (eg struct fd) to 'work'.

> I recall it was being discussed that, for the MMIO use case, it is desirable
> to have this defined in such way that one single instruction is generated for
> an aligned access of small-enough integer primitive.
> 
> This is exactly the same situation in C too. If you have a volatile struct load
> then Clang actually generates a volatile memcpy for you, and it can tear.

It could just be LLVM is broken and needs fixing in this case.