Merge pull request #43 from antoyo/update/cg_gcc_page

Update the page for rustc_codegen_gcc

Author: ojeda

src/rustc_codegen_gcc.md

@@ -8,14 +8,14 @@ Rust for Linux can be compiled with `rustc_codegen_gcc` which allows to have a G
 
 ## Building `rustc_codegen_gcc` and the sysroot
 
-The following have been tested with GCC commit [b334f15ed21c95868059a16484a1948be08e26a3](https://github.com/antoyo/gcc/commit/b334f15ed21c95868059a16484a1948be08e26a3) and `rustc_codegen_gcc` commit [c6bc7ecd65046ee502118664f42637ca318cdfb5](https://github.com/rust-lang/rustc_codegen_gcc/commit/c6bc7ecd65046ee502118664f42637ca318cdfb5).
-
-Follow the build instructions [here](https://github.com/rust-lang/rustc_codegen_gcc#building).
+Follow the build instructions [here](https://github.com/rust-lang/rustc_codegen_gcc#quick-start).
 
 ## Building Rust for Linux
 
 Follow the [Rust for Linux Quick Start instructions](https://docs.kernel.org/rust/quick-start.html) with a few changes as explained below.
 
+First, disable `MITIGATION_RETPOLINE` in `menuconfig` at: Mitigations for CPU vulnerabilities -> Avoid speculative indirect branches in kernel.
+
 Since the GCC codegen might not work on every nightly version (that should soon be fixed now that we run some tests in the Rust CI), we're going to use [the same nightly version as the GCC codegen](https://github.com/rust-lang/rustc_codegen_gcc/blob/master/rust-toolchain) instead of using the version recommended by Rust for Linux:
 
 ```sh
@@ -25,14 +25,14 @@ rustup override set nightly-2023-10-21 # Adjust to the version used by the GCC c
 Now, you need to set some variables to build Rust for Linux with the GCC codegen (do not forget to adjust your path to `rustc_codegen_gcc`):
 
 ```sh
-make -j20 KRUSTFLAGS="-Zcodegen-backend=/path/to/rustc_codegen_gcc/target/debug/librustc_codegen_gcc.so \
-    --sysroot /path/to/rustc_codegen_gcc/build_sysroot/sysroot" \
+make -j20 KRUSTFLAGS="-Zcodegen-backend=/path/to/rustc_codegen_gcc/target/debug/librustc_codegen_gcc.so" \
     HOSTRUSTFLAGS="-Zcodegen-backend=/path/to/rustc_codegen_gcc/target/debug/librustc_codegen_gcc.so \
     --sysroot /path/to/rustc_codegen_gcc/build_sysroot/sysroot -Clto=no"
 ```
 
-Since we use a different nightly version, you might need to adjust the code of the `alloc` crate built by Rust for Linux.
-You'll see some errors when running the above command in this case.
+## CI
+
+We have a [repo](https://github.com/Rust-for-Linux/ci-rustc_codegen_gcc) that runs some tests of Rust for Linux compiled with `rustc_codegen_gcc`.
 
 ## Troubleshooting
 
@@ -49,4 +49,4 @@ That should give you the correct error, which could be one of those:
 ## Contact
 
 Please contact Antoni Boucher (antoyo) on
-[IRC](https://web.libera.chat/#rustc_codegen_gcc).
+[Matrix](https://matrix.to/#/#rustc_codegen_gcc:matrix.org) or post a message on [Zulip](https://rust-lang.zulipchat.com/#narrow/channel/386786-rustc-codegen-gcc).