See Arnd's message at https://lore.kernel.org/rust-for-linux/0c844b70-19c7-4b14-ba29-fc99ae0d69f0@app.fastmail.com/:

Rihgt, I tried this last week when it came up first, removing the
'volatile' annotations in the asm-generic/io.h header and then
all the ones that caused build regressions on arm/arm64/x86
randconfig and allmodconfig builds. This patch is a little
longer than my original version as I did run into a few
regressions later.

As far as I can tell, none of these volatile annotations have
any actual effect, and most of them date back to ancient kernels
where this may have been required.

Leaving it out of the rust interface is clearly the right way,
and it shouldn't be too hard to upstream the changes below
when we need to, but I also don't see any priority to send these.
If anyone wants to help out, I can send them the whole patch.

Cleaning these would make the signatures of our Rust-side helpers match the C side.