linux-kernel - [PATCH 1/1] Kbuild, clang: add option for choosing a ThinLTO cache directory

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Date:   Mon, 12 Jul 2021 11:10:04 +0000
From:   Tor Vic <torvic9@...lbox.org>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        masahiroy@...nel.org, Nathan Chancellor <nathan@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        keescook@...omium.org
Cc:     linux-kbuild@...r.kernel.org,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: [PATCH 1/1] Kbuild, clang: add option for choosing a ThinLTO cache
 directory

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.

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@...lbox.org>
---
 Makefile                  |  5 +++--
 arch/Kconfig              | 10 ++++++++++
 scripts/Makefile.lib      |  4 ++++
 scripts/Makefile.modfinal |  4 ++++
 4 files changed, 21 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
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 10950559b223..bca87a6aa35b 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -197,6 +197,10 @@ endif
 part-of-module = $(if $(filter $(basename $@).o, $(real-obj-m)),y)
 quiet_modtag = $(if $(part-of-module),[M],   )

+ifdef CONFIG_LTO_CLANG_THIN
+KBUILD_LDFLAGS	+=
--thinlto-cache-dir=$(thinlto-dir)$(extmod-prefix).thinlto-cache
+endif
+
 modkern_cflags =                                          \
 	$(if $(part-of-module),                           \
 		$(KBUILD_CFLAGS_MODULE) $(CFLAGS_MODULE), \
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 5e9b8057fb24..ab0d72e21318 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -35,6 +35,10 @@ ifdef CONFIG_LTO_CLANG
 # avoid a second slow LTO link
 prelink-ext := .lto

+ifdef CONFIG_LTO_CLANG_THIN
+KBUILD_LDFLAGS	+=
--thinlto-cache-dir=$(thinlto-dir)$(extmod-prefix).thinlto-cache
+endif # CONFIG_LTO_CLANG_THIN
+
 # ELF processing was skipped earlier because we didn't have native code,
 # so let's now process the prelinked binary before we link the module.

-- 
2.32.0