[RFC] Using Clang (ThinLTO) for the default kernel in the cachyos repository

[ptr1337]

Hi,

Im considering to use the ThinLTO for the "linux-cachyos" as default in the cachyos repository.
Clang built kernel has been pretty stabilized over the years and is also used by Google in Android as default, together with ThinLTO.

The Clang built Kernel provides as a bunch of benefits, like:

• Rust support (could be interesting for using https://www.phoronix.com/news/Linux-6.12-DRM-Panic-QR-Code)
• Better Performance --> https://www.phoronix.com/review/clang-lto-kernel

There are some outstanding issues, which needs to be discussed/fixed:

• nvidia-open-dkms with ThinLTO does not boot, fix can be found here: dkms: fix building nvidia open kernel modules against clang and thin/full lto compiled kernel dell/dkms#417 CachyOS/CachyOS-PKGBUILDS@f402040
• Find other dkms modules, which are not compatible
• The maintenance of the archlinux clang toolchain
• Test kCFI/FineIBT for the kernel and its impact on performance and modules - Breaks NVIDIA modules

This issue is for general discussions and fiddling out the pending issue, which needs to be resolved.
Also, we should consider providing the default kernel built with GCC too, with a "linux-cachyos-gcc" variant or equal, but this will be only used for the cachyos repository.

known problematic DKMS Modules:

• vmware-workstationn (Maybe we can put this at compilation time, like we do with zfs/nvidia) https://paste.soulharsh007.dev/p/5tY8t6.log - workaround available here: Add vmware-workstation modules with Clang build fixes CachyOS-PKGBUILDS#320 can not be distributed, due license
• nvidia-390xx-dkms Fixed by: Add nvidia 390xx modules with Clang build fixes CachyOS-PKGBUILDS#318
• nvidia-470xx-dkms Fixed by: Add nvidia 470xx modules with Clang build fixes CachyOS-PKGBUILDS#319
• 8192cu-dkms - https://paste.cachyos.org/p/6059efb.c
• rtl8812au-dkms - https://paste.cachyos.org/p/385e3f2.txt
• rtl8821cu-morrownr-dkms-git - https://paste.cachyos.org/p/7545b73.txt
• rtl88xxau-aircrack-dkms-git - https://paste.cachyos.org/p/2fe7c0c.log - mainly ARM project, ignoring.

Benchmarking

We should benchmark the ThinLTO kernel compared to the GCC quite much.
There will be a bunch of cachyos-benchmarker tests done, as well as a bigger phoronix test suite.

Aditionally we should test using different marches, e.g v3/znver4 if there are any regressions found

[ventureoo]

DKMS probably will not be such a priority issue since we already mostly rely on NVIDIA/v4l2loopback modules that are already pre-built for our kernel.

[ptr1337]

vmware-workstation has problems with Clang built kernels.

According log it fails because of Wstrict-prototypes . Passing to the dkms.conf of it -Wno-strict-prototypes should fix it there.

[ventureoo]

Done with fixes for issues with NVIDIA 390xx modules: CachyOS/CachyOS-PKGBUILDS#318

[ptr1337]

We could also test and think about enabling kCFI and FineIBT for the kernel as default, when switching to the LLVM built kernels.
This needs to be tested as well as benchmarked.

kCFI should have a smaller performance hit compared to CFI, but it should be still present.
This would increase the security of the kernel.

[ptr1337]

We could also test and think about enabling kCFI and FineIBT for the kernel as default, when switching to the LLVM built kernels. This needs to be tested as well as benchmarked.

kCFI should have a smaller performance hit compared to CFI, but it should be still present. This would increase the security of the kernel.

kCFI appears to break the NVIDIA module. So, this can not be used yet. Same for FineIBT.

[shelterx]

vmware-workstation has problems with Clang built kernels.

According log it fails because of Wstrict-prototypes . Passing to the dkms.conf of it -Wno-strict-prototypes should fix it there.

I usually patch the -Werror line away when I compile my own kernels, the warning comes from /lib/modules/version/build/scripts/Makefile.extrawarn
KBUILD_CFLAGS += -Werror=strict-prototypes

Removing it doesn't seem to cause any issues.

But for cachyos repro kernels it needs some workaround.

EDIT:
Entire post. :)

[1Naim]

diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 6c23c6af797f..59b2e19416af 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -31,9 +31,7 @@ endif
 # certain optimization flags it knows it has not implemented.
 # Make it behave more like gcc by erroring when these flags are encountered
 # so they can be implemented or wrapped in cc-option.
-CLANG_FLAGS	+= -Werror=unknown-warning-option
 CLANG_FLAGS	+= -Werror=ignored-optimization-argument
 CLANG_FLAGS	+= -Werror=option-ignored
-CLANG_FLAGS	+= -Werror=unused-command-line-argument
 KBUILD_CPPFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 1d13cecc7cc7..ec97275c8852 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -12,7 +12,6 @@ KBUILD_CFLAGS += -Wundef
 KBUILD_CFLAGS += -Werror=implicit-function-declaration
 KBUILD_CFLAGS += -Werror=implicit-int
 KBUILD_CFLAGS += -Werror=return-type
-KBUILD_CFLAGS += -Werror=strict-prototypes
 KBUILD_CFLAGS += -Wno-format-security
 KBUILD_CFLAGS += -Wno-trigraphs
 KBUILD_CFLAGS += $(call cc-disable-warning,frame-address,)
@@ -68,9 +67,6 @@ KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
 # Prohibit date/time macros, which would make the build non-deterministic
 KBUILD_CFLAGS += -Werror=date-time
 
-# enforce correct pointer usage
-KBUILD_CFLAGS += $(call cc-option,-Werror=incompatible-pointer-types)
-
 # Require designated initializers for all marked structures
 KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init)

Applying this diff to the kernel makes all found incompatible dkms modules except rtl88xxau-aircrack-dkms-git to build and install successfully.

[1Naim]

Looking at https://github.com/aircrack-ng/rtl8812au, it seems that it is mainly targeting ARM/ARM64 devices. Therefore it is out of scope for us.

[ptr1337]

@1Naim
We can think about this applying the patchset, when if thinlto then source + ...
This would avoid patching it also for GCC Kernels.

[1Naim]

We can do a rollout first to the LTO kernels with a 2-4 week grace period before we make it the default.

[ms178]

Please don't forget the problems with the winesync-dkms module on newer LTO-Kernels (see: dell/dkms#439). This is a winesync problem though but shows when a modern LTO-Kernel is used. The following changes to winesync.c and winesync.h are needed to get it compiled properly with newer LTO-Kernels, see the following with more details: ms178/archpkgbuilds@ee01c7f

[ptr1337]

Please don't forget the problems with the winesync-dkms module on newer LTO-Kernels (see: dell/dkms#439). This is a winesync problem though but shows when a modern LTO-Kernel is used. The following changes to winesync.c and winesync.h are needed to get it compiled properly with newer LTO-Kernels, see the following with more details: ms178/archpkgbuilds@ee01c7f

Thats not reproduceable on llvm18. Also, we have dropped winesync support, therefore not a thing

[ms178]

Please don't forget the problems with the winesync-dkms module on newer LTO-Kernels (see: dell/dkms#439). This is a winesync problem though but shows when a modern LTO-Kernel is used. The following changes to winesync.c and winesync.h are needed to get it compiled properly with newer LTO-Kernels, see the following with more details: ms178/archpkgbuilds@ee01c7f

Thats not reproduceable on llvm18. Also, we have dropped winesync support, therefore not a thing

As mentioned in the dkms issue, it is also relevant for NTSync although with a different modpost error. Please keep it in mind as sooner or later it might become a problem with LLVM-19+.

[ptr1337]

Please don't forget the problems with the winesync-dkms module on newer LTO-Kernels (see: dell/dkms#439). This is a winesync problem though but shows when a modern LTO-Kernel is used. The following changes to winesync.c and winesync.h are needed to get it compiled properly with newer LTO-Kernels, see the following with more details: ms178/archpkgbuilds@ee01c7f

Thats not reproduceable on llvm18. Also, we have dropped winesync support, therefore not a thing

As mentioned in the dkms issue, it is also relevant for NTSync although with a different modpost error. Please keep it in mind as sooner or later it might become a problem with LLVM-19+.

That is neither an issue, since NTSync is shipped inside the kernel.

[1Naim]

Please don't forget the problems with the winesync-dkms module on newer LTO-Kernels

Well, the winesync module hasn't been updated for 2 years now. It is very much deprecated and abandoned at this point. We also do not ship it anymore as mentioned by @ptr1337.

it is also relevant for NTSync although with a different modpost error

We will tackle it once Arch has updated their LLVM toolchain. I'm sure it will cause build problems when that happens if there are no updates from upstream we need to patch it.

[ptr1337]

Benchmarked Full vs ThinLTO:

All in all, we can likely keep ThinLTO

[TheWyn]

Why not go all the way with full lto?

[1Naim]

ThinLTO is marginally better.

edit: You can also say thay its margin of error. In that case, there would also be no reason to use Full LTO.

[TheWyn]

Oh I see

[ptr1337]

From my side, I think we are mostly ready for this. We could start rolling this out on the Zen4 Repository first, and then proceed over with the v4/v3 repository after around one week (maybe with 6.11.2)

[1Naim]

IMO, we should target for 6.11.2 for all repos. Either that or wait a few days after ISO release then push to znver4.