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SubjectRe: [PATCH v2 1/1] Kbuild, clang: add option for choosing a ThinLTO cache directory
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On 14.07.21 02:46, Masahiro Yamada wrote:
> On Wed, Jul 14, 2021 at 1:41 AM Tor Vic <torvic9@mailbox.org> wrote:
>>
>> On some distros and configurations, it might be useful to allow for
>> specifying a directory where Clang stores its ThinLTO cache.
>>
>> More specifically, when building the VirtualBox extramodules on Arch with
>> its proper 'makepkg' build system and DKMS, against an already installed
>> ThinLTO kernel, the build fails because it tries to create the ThinLTO
>> cache in a directory that is not user-writable.
>
>
> Again, I do not understand.

Example:
1. build and install a kernel+headers with Arch's own makepkg/pacman tools.
2. at the end of the install, dkms is automatically called to rebuild
any out-of-tree modules like e.g. VirtualBox.
3. dkms fails because it tries to store the thinlto cache in a
directory that it not user-writable (very possibly in the installed
kernel headers directory under /usr/lib).
4. By explicitly selecting a place for the cache, e.g. /tmp, the dkms
module rebuild succeeds.

>
> Is this fixing the root cause?

If only I knew. I have very limited skills, and no IT or coding
background. It's possible that this can be fixed also on dkms but
I'm not sure.
Currently, dkms does not do a compiler check (it only seems to know
gcc) and it will fail unless explicitly told to use clang. e.g.
by specifying 'CC=clang LD=ld.lld" etc.
This has been reported to them.

>
> To me, it looks like
> "Anyway, this worked for me" patch.

At least one other person reported success by moving the cache to
another place (see link).
Admittedly, it looks the same way to me, I just wanted to get things
to work and it's likely that there are better solutions to this
problem, but AFAIR no one came forward.

>
> Besides, 'make clean' will never clean up the
> cache directory.

I can have a look at it again.

>
>
>
>
>
>> A similar problem has been reported with openSUSE's OBS build system.
>>
>> Add a Kconfig option that allows users to choose a directory in which
>> Clang's ThinLTO can store its cache.
>>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/1104
>> Signed-off-by: Tor Vic <torvic9@mailbox.org>
>> ---
>> Changes from v1 to v2: remove unneeded changes in scripts/Makefile
>>
>> Makefile | 5 +++--
>> arch/Kconfig | 10 ++++++++++
>> 2 files changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index c3f9bd191b89..472bc8bfff03 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -932,7 +932,8 @@ endif
>> ifdef CONFIG_LTO_CLANG
>> ifdef CONFIG_LTO_CLANG_THIN
>> CC_FLAGS_LTO := -flto=thin -fsplit-lto-unit
>> -KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod_prefix).thinlto-cache
>> +export thinlto-dir = $(if
>> $(CONFIG_LTO_CLANG_THIN_CACHEDIR),$(CONFIG_LTO_CLANG_THIN_CACHEDIR)/)
>> +KBUILD_LDFLAGS +=
>> --thinlto-cache-dir=$(thinlto-dir)$(extmod_prefix).thinlto-cache
>> else
>> CC_FLAGS_LTO := -flto
>> endif
>> @@ -1728,7 +1729,7 @@ PHONY += compile_commands.json
>>
>> clean-dirs := $(KBUILD_EXTMOD)
>> clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers
>> $(KBUILD_EXTMOD)/modules.nsdeps \
>> - $(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache
>> + $(KBUILD_EXTMOD)/compile_commands.json
>> $(thinlto-dir)$(KBUILD_EXTMOD)/.thinlto-cache
>>
>> PHONY += help
>> help:
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 129df498a8e1..19e4d140e12a 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -696,6 +696,16 @@ config LTO_CLANG_THIN
>> https://clang.llvm.org/docs/ThinLTO.html
>>
>> If unsure, say Y.
>> +
>> +config LTO_CLANG_THIN_CACHEDIR
>> + string "Clang ThinLTO cache directory"
>> + depends on LTO_CLANG_THIN
>> + default ""
>> + help
>> + This option allows users to choose a directory that stores
>> + Clang's ThinLTO cache.
>> + Leave empty for default.
>> +
>> endchoice
>>
>> config ARCH_SUPPORTS_CFI_CLANG
>> --
>> 2.32.0
>>
>
>

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