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[37.6.1.105]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-35f1d74b065sm5642212f8f.84.2024.06.10.13.35.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Jun 2024 13:35:49 -0700 (PDT) Date: Mon, 10 Jun 2024 23:29:36 +0300 From: Manos Pitsidianakis To: Pierrick Bouvier , qemu-devel@nongnu.org Cc: Stefan Hajnoczi , Mads Ynddal , Paolo Bonzini , Peter Maydell , Alex Benn=?UTF-8?B?w6k=?= e , "Daniel P. Berrang=?UTF-8?B?w6k=?= " , Marc-Andr=?UTF-8?B?w6kg?=Lureau , Thomas Huth , Markus Armbruster , Philippe Mathieu-Daud=?UTF-8?B?w6kg?=, Zhao Liu , Gustavo Romero Subject: Re: [RFC PATCH v1 0/6] Implement ARM PL011 in Rust User-Agent: meli 0.8.6 References: In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=utf-8; format=flowed Received-SPF: pass client-ip=2a00:1450:4864:20::330; envelope-from=manos.pitsidianakis@linaro.org; helo=mail-wm1-x330.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, 10 Jun 2024 22:37, Pierrick Bouvier wrote: >Hello Manos, > >On 6/10/24 11:22, Manos Pitsidianakis wrote: >> Hello everyone, >> >> This is an early draft of my work on implementing a very simple device, >> in this case the ARM PL011 (which in C code resides in hw/char/pl011.c >> and is used in hw/arm/virt.c). >> >> The device is functional, with copied logic from the C code but with >> effort not to make a direct C to Rust translation. In other words, do >> not write Rust as a C developer would. >> >> That goal is not complete but a best-effort case. To give a specific >> example, register values are typed but interrupt bit flags are not (but >> could be). I will leave such minutiae for later iterations. >> >> By the way, the wiki page for Rust was revived to keep track of all >> current series on the mailing list https://wiki.qemu.org/RustInQemu >> >> a #qemu-rust IRC channel was also created for rust-specific discussion >> that might flood #qemu >> > >Excellent work, and thanks for posting this RFC! > >IMHO, having patches 2 and 5 splitted is a bit confusing, and exposing >(temporarily) the generated.rs file in patches is not a good move. >Any reason you kept it this way? That was my first approach, I will rework it on the second version. The generated code should not exist in committed code at all. It was initally tricky setting up the dependency orders correctly, so I first committed it and then made it a dependency. > >Maybe it could be better if build.rs file was *not* needed for new >devices/folders, and could be abstracted as a detail of the python >wrapper script instead of something that should be committed. That'd mean you cannot work on the rust files with a LanguageServer, you cannot run cargo build or cargo check or cargo clippy, etc. That's why I left the alternative choice of including a manually generated bindings file (generated.rs.inc) > >Having a simple rust/pl011/meson.build is nice and good taste! > >> ------------------------------------------------------------------------ >> A request: keep comments to Rust in relation to the QEMU project and no >> debates on the merits of the language itself. These are valid concerns, >> but it'd be better if they were on separate mailing list threads. >> ------------------------------------------------------------------------ >> >> Table of contents: [TOC] >> >> - How can I try it? [howcanItryit] >> - What are the most important points to focus on, at this point? >> [whatarethemostimportant] >> - What are the issues with not using the compiler, rustc, directly? >> [whataretheissueswith] >> 1. Tooling >> 2. Rust dependencies >> >> - Should QEMU use third-party dependencies? [shouldqemuusethirdparty] >> - Should QEMU provide wrapping Rust APIs over QEMU internals? >> [qemuprovidewrappingrustapis] >> - Will QEMU now depend on Rust and thus not build on my XYZ platform? >> [qemudependonrustnotbuildonxyz] >> - How is the compilation structured? [howisthecompilationstructured] >> - The generated.rs rust file includes a bunch of junk definitions? >> [generatedrsincludesjunk] >> - The staticlib artifact contains a bunch of mangled .o objects? >> [staticlibmangledobjects] >> >> How can I try it? >> ================= >> [howcanItryit] Back to [TOC] >> >> Hopefully applying this patches (or checking out `master` branch from >> https://gitlab.com/epilys/rust-for-qemu/ current commit >> de81929e0e9d470deac2c6b449b7a5183325e7ee ) >> >> Tag for this RFC is rust-pl011-rfc-v1 >> >> Rustdoc documentation is hosted on >> >> https://rust-for-qemu-epilys-aebb06ca9f9adfe6584811c14ae44156501d935ba4.gitlab.io/pl011/index.html >> >> If `cargo` and `bindgen` is installed in your system, you should be able >> to build qemu-system-aarch64 with configure flag --enable-rust and >> launch an arm virt VM. One of the patches hardcodes the default UART of >> the machine to the Rust one, so if something goes wrong you will see it >> upon launching qemu-system-aarch64. >> >> To confirm it is there for sure, run e.g. info qom-tree on the monitor >> and look for x-pl011-rust. >> >> >> What are the most important points to focus on, at this point? >> ============================================================== >> [whatarethemostimportant] Back to [TOC] >> >> In my opinion, integration of the go-to Rust build system (Cargo and >> crates.io) with the build system we use in QEMU. This is "easily" done >> in some definition of the word with a python wrapper script. >> >> What are the issues with not using the compiler, rustc, directly? >> ----------------------------------------------------------------- >> [whataretheissueswith] Back to [TOC] >> >> 1. Tooling >> Mostly writing up the build-sys tooling to do so. Ideally we'd >> compile everything without cargo but rustc directly. >> >> If we decide we need Rust's `std` library support, we could >> investigate whether building it from scratch is a good solution. This >> will only build the bits we need in our devices. >> > 2. Rust dependencies >> We could go without them completely. I chose deliberately to include >> one dependency in my UART implementation, `bilge`[0], because it has >> an elegant way of representing typed bitfields for the UART's >> registers. >> >> [0]: Article: https://hecatia-elegua.github.io/blog/no-more-bit-fiddling/ >> Crates.io page: https://crates.io/crates/bilge >> Repository: https://github.com/hecatia-elegua/bilge >> >> Should QEMU use third-party dependencies? >> ----------------------------------------- >> [shouldqemuusethirdparty] Back to [TOC] >> >> In my personal opinion, if we need a dependency we need a strong >> argument for it. A dependency needs a trusted upstream source, a QEMU >> maintainer to make sure it us up-to-date in QEMU etc. >> >> We already fetch some projects with meson subprojects, so this is not a >> new reality. Cargo allows you to define "locked" dependencies which is >> the same as only fetching specific commits by SHA. No suspicious >> tarballs, and no disappearing dependencies a la left-pad in npm. >> > >As a complement to this, and for other readers, in more than having a >lock file (fixing version you use), cargo crates system is designed to >be immutable (see: https://crates.io/policies), and it means there is a >strong guarantee that a published package will stay there, to the >opposite of npm, pypi, or most of other similar systems. > >"Crate deletion by their owners is not possible to keep the registry as >immutable as possible." > >I believe this is a *key* feature of Rust ecosystem and should be >emphasized regarding the policy for Rust dependencies to come. > >> However, I believe it's worth considering vendoring every dependency by >> default, if they prove to be few, for the sake of having a local QEMU >> git clone buildable without network access. >> > >I would not be in favor to vendor all dependencies. Beyond the "offline >build" scenario, it has only downsides. > >Sure, we should really debate before introducing a new dependency, but >the technical difficulty to mirror its sources and dependencies should >not be an argument for or against it. > >What will happen the day we want to introduce something bigger than a >simple dependency? (let's say "serde" for instance) Yes, vendor-the-world is a different topic than vendor e.g. two crates such as the dependencies I'm using here. > >> Should QEMU provide wrapping Rust APIs over QEMU internals? >> ----------------------------------------------------------- >> [qemuprovidewrappingrustapis] Back to [TOC] >> >> My personal opinion is no, with the reasoning being that QEMU internals >> are not documented or stable. However I do not see why creating stable >> opt-in interfaces is bad. It just needs someone to volunteer to maintain >> it and ensure there are no breakages through versions. >> >> Will QEMU now depend on Rust and thus not build on my XYZ platform? >> ------------------------------------------------------------------- >> [qemudependonrustnotbuildonxyz] Back to [TOC] >> >> No, worry about this in some years if this experiment takes off. Rust >> has broad platform support and is present in most distro package >> managers. In the future we might have gcc support for it as well. >> >> For now, Rust will have an experimental status, and will be aimed to >> those who wish to try it. I leave it to the project leaders to make >> proper decisions and statements on this if necessary. >> >> >> How is the compilation structured? >> ================================== >> [howisthecompilationstructured] Back to [TOC] >> >> First, a meson target that runs `bindgen` on a bunch of header files >> (defined in `rust/wrapper.h`) is created as a target and as a dependency >> for any rust hardware device that needs it. You can see the generated >> bindings by running >> >> ninja generated.rs >> >> inside your build directory. >> >> The devices are defined as dictionaries in rust/meson.build because they >> depend on the bindgen dependency, which is available much later in the >> meson process (when the static qemu lib and target emulator executables >> are defined). >> >> A cargo wrapper python script under scripts/ exists to build the crate >> library, by providing the path to the generated.rs bindings via the >> environment. Then, the qemu-system-aarch64 binary links against the >> staticlib archive (i.e. libpl011.a) >> >> The generated.rs rust file includes a bunch of junk definitions? >> ================================================================ >> [generatedrsincludesjunk] Back to [TOC] >> >> Yes, bindgen allows you to block certain types and identifiers from >> being generated but they are simply too many. I have trimmed some of the >> fat but vast improvements can be made. >> >> The staticlib artifact contains a bunch of mangled .o objects? >> ============================================================== >> [staticlibmangledobjects] Back to [TOC] >> >> Yes, until we compile without the `std` module library or we compile it >> manually instead of linking it, we will have some junk in it. >> > >Besides the size aspect, which potential advantage would there be to >switch to no_std? >We don't build a bare metal or kernel binary here, so why introduce this >restriction willingly? We'll see that as we progress. Might enable more platform support, for example. I have no definite answers here. Also, I know binary bloat is a big complaint from people with dislike of Rust, so I pre-emptively addressed it.