Can I have Rust, BTF, and LTO enabled at the same time?

[leyvi]

I'm asking because it looks like I can't, but maybe there's something I don't know. I'm asking because I'm starting to get into AutoFDO profiling, which is supposed to benefit from LTO.

[Hu]

Why do you think you cannot have them all? What error messages did you see when you tried? What document did you read that told you that this cannot work?

[leyvi]

Why do you think you cannot have them all? What error messages did you see when you tried? What document did you read that told you that this cannot work?

In menuconfig, it says that CONFIG_RUST depends on a long list of things being present or absent. Among other things, it would seem that I can't have debug symbols and LTO, among other things. See "Depends on:" for CONFIG_RUST.

[skellr]

In menuconfig, it says that CONFIG_RUST depends on a long list of things being present or absent. Among other things, it would seem that I can't have debug symbols and LTO, among other things. See "Depends on:" for CONFIG_RUST.

You want to enable the rust-src use flag for rust. The others may already be set by default:

Code: Select all

dev-lang/rust rust-src rustfmt clippy

Then check to see if you have the requirements to use rust:

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make LLVM=1 rustavailable

You want debug but not split debug for CONFIG_DEBUG_INFO_BTF:
!DEBUG_INFO_SPLIT [=n] && !DEBUG_INFO_REDUCED [=n]

and pahole: PAHOLE_VERSION [=131] >=122 && !HEXAGON

[leyvi]

Hang on; CONFIG_RUST depends on CONFIG_GENDWARFKSYMS (if CONFIG_MODVERSIONS is set, which is needed by CONFIG_RUST through CONFIG_EXTENDED_MODVERSIONS) which can't be enabled at the same time as CONFIG_LTO.

It also looks like DEBUG_INFO_BTF and CONFIG_LTO cannot both be enabled if CONFIG_RUST is enabled:

Code: Select all

(!DEBUG_INFO_BTF [=y] || PAHOLE_HAS_LANG_EXCLUDE [=y] && !LTO [=n])

So I can't have all 3.

This seems unintuitive to me; why isn't this possible? Am I mistaken?

[Hu]

git blame on the blocking depends on line leads me to commit 5daa0c35a1f0e7a6c3b8ba9cb721e7d1ace6e619:

Code: Select all

author Matthew Maurer <mmaurer@google.com> Wed Jan 8 23:35:08 2025 +0000
committer Miguel Ojeda <ojeda@kernel.org> Tue Mar 11 20:11:09 2025 +0100

rust: Disallow BTF generation with Rust + LTO

The kernel cannot currently self-parse BTF containing Rust debug
information. pahole uses the language of the CU to determine whether to
filter out debug information when generating the BTF. When LTO is
enabled, Rust code can cross CU boundaries, resulting in Rust debug
information in CUs labeled as C. This results in a system which cannot
parse its own BTF.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Cc: stable@vger.kernel.org
Fixes: c1177979af9c ("btf, scripts: Exclude Rust CUs with pahole")
Link: https://lore.kernel.org/r/20250108-rust-btf-lto-incompat-v1-1-60243ff6d820@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Does that log message answer your question?

[skellr]

Hang on; CONFIG_RUST depends on CONFIG_GENDWARFKSYMS (if CONFIG_MODVERSIONS is set, which is needed by CONFIG_RUST through CONFIG_EXTENDED_MODVERSIONS) which can't be enabled at the same time as CONFIG_LTO.

It also looks like DEBUG_INFO_BTF and CONFIG_LTO cannot both be enabled if CONFIG_RUST is enabled:

Code: Select all

(!DEBUG_INFO_BTF [=y] || PAHOLE_HAS_LANG_EXCLUDE [=y] && !LTO [=n])

So I can't have all 3.

This seems unintuitive to me; why isn't this possible? Am I mistaken?

Ohh, no. I was mistaken.
I just went though the config enabling CONFIG_RUST and then enabling CONFIG_DEBUG_INFO_BTF and didn't notice that it disabled CONFIG_RUST

Sorry about that.

[leyvi]

git blame on the blocking depends on line leads me to commit 5daa0c35a1f0e7a6c3b8ba9cb721e7d1ace6e619:

Code: Select all

author Matthew Maurer <mmaurer@google.com> Wed Jan 8 23:35:08 2025 +0000
committer Miguel Ojeda <ojeda@kernel.org> Tue Mar 11 20:11:09 2025 +0100

rust: Disallow BTF generation with Rust + LTO

The kernel cannot currently self-parse BTF containing Rust debug
information. pahole uses the language of the CU to determine whether to
filter out debug information when generating the BTF. When LTO is
enabled, Rust code can cross CU boundaries, resulting in Rust debug
information in CUs labeled as C. This results in a system which cannot
parse its own BTF.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Cc: stable@vger.kernel.org
Fixes: c1177979af9c ("btf, scripts: Exclude Rust CUs with pahole")
Link: https://lore.kernel.org/r/20250108-rust-btf-lto-incompat-v1-1-60243ff6d820@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Does that log message answer your question?

Yes, thank you.

I should probably figure out how to use git blame.