Re: [RFC PATCH v2 3/5] rust: add PL011 device model

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jun 19, 2024 at 06:43:01PM +0200, Paolo Bonzini wrote:
> On 6/19/24 07:34, Richard Henderson wrote:
> > First silly question: how much of this is boiler plate that gets moved
> > the moment that the second rust subdirectory is added?
> 
> If my suggestion at https://lore.kernel.org/qemu-devel/CABgObfaP7DRD8dbSKNmUzhZNyxeHWO0MztaW3_EFYt=vf6SbzA@mail.gmail.com/
> works, we'd have only two directories that have a Cargo.toml in it.  For
> example it could be rust/qemu/ (common code) and rust/hw/ (code that depends
> on Kconfig).
> 
> I think we can also have a rust/Cargo.toml file as in
> https://doc.rust-lang.org/cargo/reference/workspaces.html#virtual-workspace,
> and then the various configuration files under rust/ will be valid for all
> subpackages.
> 
> > > +[build]
> > > +rustflags = ["-Crelocation-model=pic", "-Ctarget-feature=+crt-static"]
> > 
> > It seems certain that this is not specific to pl011, and will be commot
> > to other rust subdirectories.  Or, given the .cargo directory name, is
> > this generated by cargo and committed by mistake?
> 
> -Crelocation-mode should be pie.  But also, I am not sure it works because I
> think it's always going to be overridden by cargo_wrapper.py? See
> https://doc.rust-lang.org/cargo/reference/config.html#buildrustflags.
> 
> (I'm not sure what +crt-static is for).
> 
> I think the generate_cfg_flags() mechanism of cargo_wrapper.py has to be
> rewritten from Python to Rust and moved to build.rs (using
> cargo::rustc-cfg).  By doing this, the cfg flags are added to whatever is in
> .cargo/config.toml, rather than replaced.
> 
> > > diff --git a/rust/pl011/.gitignore b/rust/pl011/.gitignore
> > > new file mode 100644
> > > index 0000000000..28a02c847f
> > > --- /dev/null
> > > +++ b/rust/pl011/.gitignore
> > > @@ -0,0 +1,2 @@
> > > +target
> > > +src/generated.rs.inc
> > 
> > Is this a symptom of generating files into the source directory and not
> > build directory?
> 
> If I understand correctly, Manos considered two possible ways to invoke
> cargo on the Rust code:
> 
> - directly, in which case you need to copy the generated source file to
> rust/pl011/src/generated.rs.inc, because cargo does not know where the build
> tree
> 
> - do everything through meson, which does the right thing because
> cargo_wrapper.py knows about the build tree and passes the information.
> 
> To avoid this, the first workflow could be adjusted so that cargo can still
> be invoked directly, _but from the build tree_, not the source tree.  For
> example configure could generate a .../build/.cargo/config.toml with
> 
>    [env]
>    MESON_BUILD_ROOT = ".../build"
> 
> (extra advantage: -Crelocation-model=pie can be added based on
> --enable-pie/--disable-pie).  configure can also create symlinks in the
> build tree for the source tree's rust/, Cargo.toml and Cargo.lock.
> 
> This allows rust/pl011/src/generated.rs (which probably will become
> something like rust/common/src/generated.rs) to be:
> 
>    include!(concat!(env!("MESON_BUILD_ROOT"), "/generated.rs.inc"));
> 
> when cargo is invoked from the build tree.
> 
> Putting everything together you'd have
> 
>    build/
>      rust/
>        .cargo/
>          config.toml   # generated by configure or meson.build
>        Cargo.toml      # workspace generated by configure or meson.build
>        Cargo.lock      # can be either linked to srctree or generated
>        qemu/           # symlink to srctree/rust/qemu
>        aarch64-softmmu-hw/
>          Cargo.toml    # generated by meson.build (*)
>          src/          # symlink to srctree/rust/hw/
>        i386-softmmu-hw/
>          Cargo.toml    # generated by meson.build
>          src/          # symlink to srctree/rust/hw/
>        generated/      # files generated by rust/generated/meson.build

If we're generating a build tree to invoke cargo on, can we then
avoid creating a completely separate dir hierarchy in the source
tree rooted at /rust, and just have rust source within our existing
hierarchy.

eg

        aarch64-softmmu-hw/
          Cargo.toml    # generated by meson.build (*)
          src/
            pl011/      # symlink to srctree/hw/p1011/


With regards,
Daniel
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