Re: [PATCH 1/2] rust/qemu-api: Add initial logging support based on C API

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/30/25 22:58, Bernhard Beschow wrote:
> A qemu_log_mask!() macro is provided which expects similar arguments as the C
> version. However, the formatting works as one would expect from Rust.
> 
> To maximize code reuse the macro is just a thin wrapper around qemu_log().
> Also, just the bare minimum of logging masks is provided which should suffice
> for the current use case of Rust in QEMU.

It's probably better to use an enum for this.  One possibility is also 
to change the #defines to a C enum, and see which enum translation of 
the several allowed by bindgen is best.

Also, while this is good for now, later on we probably want to 
reimplement logging at a lower level via the std::fmt::Write trait.  But 
that's just for efficiency and your macro is indeed good enough to 
define what the API would look like.  Right now I have a project for 
GSoC that will look at that, and the student can look into it later on.

This means answering the following two questions:

- the mapping the LOG_* constants into Rust

- whether to keep the "qemu" prefix for the API (personal opinion: no)

- whether it makes sense to add more macros such as log_guest_error! or 
log_unimp! for the most common LOG_* values

> +#[macro_export]
> +macro_rules! qemu_log_mask {
> +    ($mask:expr, $fmt:expr $(, $args:expr)*) => {{

Looking at https://doc.rust-lang.org/std/macro.write.html they just use 
$($arg:tt)* for what is passed to format_args! (or in your case 
format!), so we can do the same here too. The main advantage is that it 
allows giving a trailing comma to qemu_log_mask!.

Paolo

> +        if unsafe {
> +            (::qemu_api::bindings::qemu_loglevel & ($mask as std::os::raw::c_int)) != 0
> +        } {
> +            let formatted_string = format!($fmt, $($args),*);
> +            let c_string = std::ffi::CString::new(formatted_string).unwrap();
> +
> +            unsafe {
> +                ::qemu_api::bindings::qemu_log(c_string.as_ptr());
> +            }
> +        }
> +    }};
> +}