path: root/Makefile
diff options
authorNadav Amit <namit@vmware.com>2018-10-03 14:30:52 -0700
committerIngo Molnar <mingo@kernel.org>2018-10-04 10:57:09 +0200
commit77b0bf55bc675233d22cd5df97605d516d64525e (patch)
treedc568685e7ab654809212470f818e966fc48be62 /Makefile
parent35e76b99ddf20405a6196bb7c9eb152675c93106 (diff)
kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs
Using macros in inline assembly allows us to work around bugs in GCC's inlining decisions. Compile macros.S and use it to assemble all C files. Currently only x86 will use it. Background: The inlining pass of GCC doesn't include an assembler, so it's not aware of basic properties of the generated code, such as its size in bytes, or that there are such things as discontiuous blocks of code and data due to the newfangled linker feature called 'sections' ... Instead GCC uses a lazy and fragile heuristic: it does a linear count of certain syntactic and whitespace elements in inlined assembly block source code, such as a count of new-lines and semicolons (!), as a poor substitute for "code size and complexity". Unsurprisingly this heuristic falls over and breaks its neck whith certain common types of kernel code that use inline assembly, such as the frequent practice of putting useful information into alternative sections. As a result of this fresh, 20+ years old GCC bug, GCC's inlining decisions are effectively disabled for inlined functions that make use of such asm() blocks, because GCC thinks those sections of code are "large" - when in reality they are often result in just a very low number of machine instructions. This absolute lack of inlining provess when GCC comes across such asm() blocks both increases generated kernel code size and causes performance overhead, which is particularly noticeable on paravirt kernels, which make frequent use of these inlining facilities in attempt to stay out of the way when running on baremetal hardware. Instead of fixing the compiler we use a workaround: we set an assembly macro and call it from the inlined assembly block. As a result GCC considers the inline assembly block as a single instruction. (Which it often isn't but I digress.) This uglifies and bloats the source code - for example just the refcount related changes have this impact: Makefile | 9 +++++++-- arch/x86/Makefile | 7 +++++++ arch/x86/kernel/macros.S | 7 +++++++ scripts/Kbuild.include | 4 +++- scripts/mod/Makefile | 2 ++ 5 files changed, 26 insertions(+), 3 deletions(-) Yay readability and maintainability, it's not like assembly code is hard to read and maintain ... We also hope that GCC will eventually get fixed, but we are not holding our breath for that. Yet we are optimistic, it might still happen, any decade now. [ mingo: Wrote new changelog describing the background. ] Tested-by: Kees Cook <keescook@chromium.org> Signed-off-by: Nadav Amit <namit@vmware.com> Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Michal Marek <michal.lkml@markovi.net> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Sam Ravnborg <sam@ravnborg.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kbuild@vger.kernel.org Link: http://lkml.kernel.org/r/20181003213100.189959-3-namit@vmware.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'Makefile')
1 files changed, 7 insertions, 2 deletions
diff --git a/Makefile b/Makefile
index 6c3da3e10f07..6c40d547513c 100644
--- a/Makefile
+++ b/Makefile
@@ -1071,7 +1071,7 @@ scripts: scripts_basic asm-generic gcc-plugins $(autoksyms_h)
# version.h and scripts_basic is processed / created.
# Listed in dependency order
-PHONY += prepare archprepare prepare0 prepare1 prepare2 prepare3
+PHONY += prepare archprepare macroprepare prepare0 prepare1 prepare2 prepare3
# prepare3 is used to check if we are building in a separate output directory,
# and if so do:
@@ -1094,7 +1094,9 @@ prepare2: prepare3 outputmakefile asm-generic
prepare1: prepare2 $(version_h) $(autoksyms_h) include/generated/utsrelease.h
-archprepare: archheaders archscripts prepare1 scripts_basic
+macroprepare: prepare1 archmacros
+archprepare: archheaders archscripts macroprepare scripts_basic
prepare0: archprepare gcc-plugins
$(Q)$(MAKE) $(build)=.
@@ -1162,6 +1164,9 @@ archheaders:
PHONY += archscripts
+PHONY += archmacros
PHONY += __headers
__headers: $(version_h) scripts_basic uapi-asm-generic archheaders archscripts
$(Q)$(MAKE) $(build)=scripts build_unifdef