[PATCH v2] kbuild: rust: add AutoFDO support

[Alice Ryhl <aliceryhl@google.com>]

This patch enables AutoFDO build support for Rust code within the Linux
kernel. This allows Rust code to be profiled and optimized based on the
profile.

The RUSTFLAGS variable was suffixed with *_AUTOFDO_CLANG to match the
naming of the config option, which is called CONFIG_AUTOFDO_CLANG.

This implementation has been verified in Android, first by inspecting
the object files and confirming that they look correct. After that,
it was verified as below:

1. Running the binderAddInts benchmark [1] with Rust Binder built as
   rust_binder.ko module, using a Pixel 9 Pro.
2. Collecting a profile on an Pixel 10 Pro XL using the app-launch
   benchmark, which starts different apps many times, on a device with
   Rust Binder as a built-in kernel module. (C Binder was not present on
   the device.)
3. Using the collected profile, run the binderAddInts benchmark again
   with Rust Binder built both as a rust_binder.ko module, and as a
   built-in kernel module.
4. In both cases, Rust Binder without AutoFDO was approximately 13%
   slower than the AutoFDO optimized version. Built-in vs .ko did not
   make a measurable performance difference.

All of the above was verified in conjuction with my helpers inlining
series [2], which confirmed that this worked correctly for helpers too
once [3] was fixed in the helpers inlining series.

Link: https://android.googlesource.com/platform/system/extras/+/920f089/tests/binder/benchmarks/binderAddInts.cpp [1]
Link: https://lore.kernel.org/r/20260203-inline-helpers-v2-0-beb8547a03c9@google.com [2]
Link: https://lore.kernel.org/r/aasPsbMEsX6iGUl8@google.com [3]
Reviewed-by: Rong Xu <xur@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Discuss RUSTFLAGS_ naming in commit message.
- Update commit message with information about additional testing that
  has now been carried out.
- Pick up tags.
- Link to v1: https://lore.kernel.org/r/20260319-autofdo-v1-1-51ee2a7290cd@google.com
---
 scripts/Makefile.autofdo | 6 +++++-
 scripts/Makefile.lib     | 3 +++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.autofdo b/scripts/Makefile.autofdo
index 1caf2457e585..3f08acab4549 100644
--- a/scripts/Makefile.autofdo
+++ b/scripts/Makefile.autofdo
@@ -3,14 +3,18 @@
 # Enable available and selected Clang AutoFDO features.
 
 CFLAGS_AUTOFDO_CLANG := -fdebug-info-for-profiling -mllvm -enable-fs-discriminator=true -mllvm -improved-fs-discriminator=true
+RUSTFLAGS_AUTOFDO_CLANG := -Zdebug-info-for-profiling -Cllvm-args=-enable-fs-discriminator=true -Cllvm-args=-improved-fs-discriminator=true
 
 ifndef CONFIG_DEBUG_INFO
   CFLAGS_AUTOFDO_CLANG += -gmlt
+  RUSTFLAGS_AUTOFDO_CLANG += -Cdebuginfo=line-tables-only
 endif
 
 ifdef CLANG_AUTOFDO_PROFILE
   CFLAGS_AUTOFDO_CLANG += -fprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -ffunction-sections
   CFLAGS_AUTOFDO_CLANG += -fsplit-machine-functions
+  RUSTFLAGS_AUTOFDO_CLANG += -Zprofile-sample-use=$(CLANG_AUTOFDO_PROFILE) -Zfunction-sections=y
+  RUSTFLAGS_AUTOFDO_CLANG += -Cllvm-args=-split-machine-functions
 endif
 
 ifdef CONFIG_LTO_CLANG_THIN
@@ -21,4 +25,4 @@ ifdef CONFIG_LTO_CLANG_THIN
   KBUILD_LDFLAGS += -plugin-opt=-split-machine-functions
 endif
 
-export CFLAGS_AUTOFDO_CLANG
+export CFLAGS_AUTOFDO_CLANG RUSTFLAGS_AUTOFDO_CLANG
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 0718e39cedda..eaddf6637669 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -123,6 +123,9 @@ ifeq ($(CONFIG_AUTOFDO_CLANG),y)
 _c_flags += $(if $(patsubst n%,, \
 	$(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(is-kernel-object)), \
 	$(CFLAGS_AUTOFDO_CLANG))
+_rust_flags += $(if $(patsubst n%,, \
+	$(AUTOFDO_PROFILE_$(target-stem).o)$(AUTOFDO_PROFILE)$(is-kernel-object)), \
+	$(RUSTFLAGS_AUTOFDO_CLANG))
 endif
 
 #

---
base-commit: 1f318b96cc84d7c2ab792fcc0bfd42a7ca890681
change-id: 20260309-autofdo-8d01e7977fed

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>