Re: [PATCH 1/3] rust: add static_call support

[Alice Ryhl <aliceryhl@google.com>]

Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Jun 06, 2024 at 09:09:00PM +0200, Miguel Ojeda wrote:
> > On Thu, Jun 6, 2024 at 7:19 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > This is absolutely unreadable gibberish -- how am I supposed to keep
> > > this in sync with the rest of the static_call infrastructure?
> > 
> > Yeah, they are macros, which look different from "normal" Rust code.
> 
> Macros like CPP ?

Yes, this patch series uses declarative macros, which are the closest
that Rust has to the C preprocessor. They are powerful, but just like
CPP, they can become pretty complicated and hard to read if you are
doing something non-trivial.

The macro_rules! block is how you define a new declarative macro.

The ($name:ident($($args:expr),* $(,)?)) part defines the arguments to
the declarative macro. This syntax means:

1. The input starts with any identifier, which we call $name.
2. Then there must be a ( token.
3. Then the $(),* part defines a repetition group. The contents inside
   the parenthesises are what is being repeated. The comma means that
   repetitions are separated by commas. The asterisk means that the
   contents may be repeated any number of times, including zero times.
   (Alternatives are + which means repeated at least once, and ? which
   means that the contents may be repeated zero or one time.)
4. The contents of the repetition group will be an expression, which we
   call $args.
5. After the last expression, we have $(,)? which means that you can
   have zero or one commas after the last expression. Rust usually
   allows trailing commas in lists, which is why I included it.
6. And finally, you must close the input with a ) token.

So for example, you might invoke the macro like this:

static_call!(tp_func_my_tracepoint(__data, &mut my_task_struct));

Here, $name will be tp_func_my_tracepoint, and the repetition group is
repeated twice, with $args first corresponding to `__data` and then to
`&mut my_task_struct` when the macro is expanded. The $(,)? group is
repeated zero times.


Inside the macro, you will see things such as:
$crate::macros::paste! { $crate::bindings:: [<__SCK__ $name >]; }

The Rust syntax for invoking a macro has an exclamation mark after the
name, so you know that $crate::macros::paste is a macro. The `paste`
macro just emits its input unchanged, except that any identifiers
between [< and >] are concatenated into a single identifier. So if $name
is my_static_key, then the above invocation of paste! emits:

	$crate::bindings::__SCK__my_static_key;

The $crate::bindings module is where the output of bindgen goes, so this
should correspond to the C symbol called __SCK__my_static_key.

> > Is there something we could do to help here? I think Alice and others
> > would be happy to explain how it works and/or help maintain it in the
> > future if you prefer.
> 
> Write a new language that looks more like C -- pretty please ? :-)
> 
> Mostly I would just really like you to just use arm/jump_label.h,
> they're inline functions and should be doable with IR, no weirdo CPP
> involved in this case.

I assume that you're referring to static_key_false here? I don't think
that function can be exposed using IR because it passes the function
argument called key as an "i" argument to an inline assembly block. Any
attempt to compile static_key_false without knowing the value of key at
compile time will surely fail to compile with the

	invalid operand for inline asm constraint 'i'

error.

Alice