Re: [RFC 1/2] rust: Introduce atomic API helpers

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Jun 14, 2024 at 11:31:33AM +0100, Mark Rutland wrote:
> On Wed, Jun 12, 2024 at 03:30:24PM -0700, Boqun Feng wrote:
> > In order to support LKMM atomics in Rust, add rust_helper_* for atomic
> > APIs. These helpers ensure the implementation of LKMM atomics in Rust is
> > the same as in C. This could save the maintenance burden of having two
> > similar atomic implementations in asm.
> > 
> > Originally-by: Mark Rutland <mark.rutland@arm.com>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> 
> FWIW, I'm happy with the concept; I have a couple of minor comments

;-)

> below.
> 
> > ---
> >  rust/atomic_helpers.h                     | 1035 +++++++++++++++++++++
> >  rust/helpers.c                            |    2 +
> >  scripts/atomic/gen-atomics.sh             |    1 +
> >  scripts/atomic/gen-rust-atomic-helpers.sh |   64 ++
> >  4 files changed, 1102 insertions(+)
> >  create mode 100644 rust/atomic_helpers.h
> >  create mode 100755 scripts/atomic/gen-rust-atomic-helpers.sh
> 
> [...]
> 
> > +#gen_proto_order_variant(meta, pfx, name, sfx, order, atomic, int, raw, arg...)
> > +gen_proto_order_variant()
> > +{
> > +	local meta="$1"; shift
> > +	local pfx="$1"; shift
> > +	local name="$1"; shift
> > +	local sfx="$1"; shift
> > +	local order="$1"; shift
> > +	local atomic="$1"; shift
> > +	local int="$1"; shift
> > +	local raw="$1"; shift
> > +	local attrs="${raw:+noinstr }"
> 
> You removed the 'raw_' atomic generation below, so you can drop the
> 'raw' parameter and the 'attrs' variable (both here and in the
> template)...
> 
> > +	local atomicname="${raw}${atomic}_${pfx}${name}${sfx}${order}"
> > +
> > +	local ret="$(gen_ret_type "${meta}" "${int}")"
> > +	local params="$(gen_params "${int}" "${atomic}" "$@")"
> > +	local args="$(gen_args "$@")"
> > +	local retstmt="$(gen_ret_stmt "${meta}")"
> > +
> > +cat <<EOF
> > +__rust_helper ${attrs}${ret}
> 
> ... e.g. you can remove '${attrs}' here.
> 
> [...]
> 
> > +grep '^[a-z]' "$1" | while read name meta args; do
> > +	gen_proto "${meta}" "${name}" "atomic" "int" "" ${args}
> > +done
> > +
> > +grep '^[a-z]' "$1" | while read name meta args; do
> > +	gen_proto "${meta}" "${name}" "atomic64" "s64" "" ${args}
> > +done
> 
> With the 'raw' parameter removed above, the '""' argument can be
> dropped.
> 

Fix all above locally.

> Any reason to not have the atomic_long_*() API? It seems like an odd
> ommision.
> 

See my reply to Peter, but there's also a more technical reason: right
now, we use core::ffi::c_long for bindgen to translate C's long. But
instead of `isize` (Rust's version of pointer-sized integer)
core::ffi::c_long is `i64` on 64bit and `i32` on 32bit. So right now,
atomic_long_add_return()'s helper signature would be (on 64bit):

	extern "C" {
	    #[link_name="rust_helper_atomic_long_add_return"]
	    pub fn atomic_long_add_return(
		i: i64,
		v: *mut atomic_long_t,
	    ) -> i64;
	}

and I would need to cast the types in order to put it in an
`AtomicIsize` method:

	impl AtomicIsize {
	    pub fn add_return(&self, i: isize) -> isize {
	        unsafe {
		    return atomic_long_add_return(i as i64, self.0.get()) as isize
		}
	    }
	}

see these two `as`s. I want to avoid handling them in a bash script ;-)

A better solution would be what Gary has:

	https://github.com/nbdd0121/linux/commit/b604a43db56f149a90084fa8aed7988a8066894b	

, which defines kernel's own ffi types and teach bindgen to pick the
right type for c_long. If we prefer script code generating to Rust macro
code generating, I will work with Gary on getting that done first and
then add atomic_long_t support unless we feel atomic_long_t support is
urgent.

Regards,
Boqun

> Mark.