Re: [PATCH 3/3] rust: pl011: Check size of state struct at compile time

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 20 Mar 2025 at 15:25, Zhao Liu <zhao1.liu@intel.com> wrote:
>
> > -use std::{ffi::CStr, ptr::addr_of_mut};
> > +use std::{ffi::CStr, mem, ptr::addr_of_mut};
>
> maybe mem::size_of (since there're 2 use cases :-))?
>
> >
> >  use qemu_api::{
> > +    bindings,
> >      chardev::{CharBackend, Chardev, Event},
> > +    static_assert,
>
> This one looks like it breaks the alphabetical ordering (this nit can be
> checked and fixed by "cargo +nightly fmt" under rust directory, which is
> like checkpatch.pl).

Yep; I put it here because I started with Paolo's v1 patch
which called the macro "const_assert", and then when v2
changed the macro name I forgot to move it later in the use {}.

> >      impl_vmstate_forward,
> >      irq::{IRQState, InterruptSource},
> >      memory::{hwaddr, MemoryRegion, MemoryRegionOps, MemoryRegionOpsBuilder},
> > @@ -124,6 +126,12 @@ pub struct PL011State {
> >      pub migrate_clock: bool,
> >  }
> >
> > +// Some C users of this device embed its state struct into their own
> > +// structs, so the size of the Rust version must not be any larger
> > +// than the size of the C one. If this assert triggers you need to
> > +// expand the padding_for_rust[] array in the C PL011State struct.
> > +static_assert!(mem::size_of::<PL011State>() <= mem::size_of::<bindings::PL011State>());
> > +
>
> maybe use qemu_api::bindings::PL011State directly? Because bindings
> contains native C structures/functions and their use should not be
> encouraged, I think it's better to 'hide' bindings (not list it at the
> beginning of the file).

Yeah, I wasn't sure what our preferred style approach is here
regarding what we "use" and what we just directly reference
(and the same in the other direction for mem::size_of vs
size_of). Is there a "normal" pattern to follow here ?

Speaking of size_of, I noticed that Rust provides both
core::mem::size_of and std::mem::size_of, and in rust/ at
the moment we have uses of both. What's the difference?

thanks
-- PMM