[PATCH v2] kbuild: rust: add AutoFDO support

Alice Ryhl
2026-03-31T10:57:57Z

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>