Commit 77b0bf55 authored by Nadav Amit's avatar Nadav Amit Committed by Ingo Molnar

kbuild/Makefile: Prepare for using macros in inline assembly code to work...

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: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarNadav Amit <namit@vmware.com>
Acked-by: default avatarMasahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: default avatarPeter 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.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 35e76b99
...@@ -1071,7 +1071,7 @@ scripts: scripts_basic asm-generic gcc-plugins $(autoksyms_h) ...@@ -1071,7 +1071,7 @@ scripts: scripts_basic asm-generic gcc-plugins $(autoksyms_h)
# version.h and scripts_basic is processed / created. # version.h and scripts_basic is processed / created.
# Listed in dependency order # 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, # prepare3 is used to check if we are building in a separate output directory,
# and if so do: # and if so do:
...@@ -1094,7 +1094,9 @@ prepare2: prepare3 outputmakefile asm-generic ...@@ -1094,7 +1094,9 @@ prepare2: prepare3 outputmakefile asm-generic
prepare1: prepare2 $(version_h) $(autoksyms_h) include/generated/utsrelease.h prepare1: prepare2 $(version_h) $(autoksyms_h) include/generated/utsrelease.h
$(cmd_crmodverdir) $(cmd_crmodverdir)
archprepare: archheaders archscripts prepare1 scripts_basic macroprepare: prepare1 archmacros
archprepare: archheaders archscripts macroprepare scripts_basic
prepare0: archprepare gcc-plugins prepare0: archprepare gcc-plugins
$(Q)$(MAKE) $(build)=. $(Q)$(MAKE) $(build)=.
...@@ -1162,6 +1164,9 @@ archheaders: ...@@ -1162,6 +1164,9 @@ archheaders:
PHONY += archscripts PHONY += archscripts
archscripts: archscripts:
PHONY += archmacros
archmacros:
PHONY += __headers PHONY += __headers
__headers: $(version_h) scripts_basic uapi-asm-generic archheaders archscripts __headers: $(version_h) scripts_basic uapi-asm-generic archheaders archscripts
$(Q)$(MAKE) $(build)=scripts build_unifdef $(Q)$(MAKE) $(build)=scripts build_unifdef
......
...@@ -236,6 +236,13 @@ archscripts: scripts_basic ...@@ -236,6 +236,13 @@ archscripts: scripts_basic
archheaders: archheaders:
$(Q)$(MAKE) $(build)=arch/x86/entry/syscalls all $(Q)$(MAKE) $(build)=arch/x86/entry/syscalls all
archmacros:
$(Q)$(MAKE) $(build)=arch/x86/kernel arch/x86/kernel/macros.s
ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s -Wa,-
export ASM_MACRO_FLAGS
KBUILD_CFLAGS += $(ASM_MACRO_FLAGS)
### ###
# Kernel objects # Kernel objects
......
/* SPDX-License-Identifier: GPL-2.0 */
/*
* This file includes headers whose assembly part includes macros which are
* commonly used. The macros are precompiled into assmebly file which is later
* assembled together with each compiled file.
*/
...@@ -115,7 +115,9 @@ __cc-option = $(call try-run,\ ...@@ -115,7 +115,9 @@ __cc-option = $(call try-run,\
# Do not attempt to build with gcc plugins during cc-option tests. # Do not attempt to build with gcc plugins during cc-option tests.
# (And this uses delayed resolution so the flags will be up to date.) # (And this uses delayed resolution so the flags will be up to date.)
CC_OPTION_CFLAGS = $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS)) # In addition, do not include the asm macros which are built later.
CC_OPTION_FILTERED = $(GCC_PLUGINS_CFLAGS) $(ASM_MACRO_FLAGS)
CC_OPTION_CFLAGS = $(filter-out $(CC_OPTION_FILTERED),$(KBUILD_CFLAGS))
# cc-option # cc-option
# Usage: cflags-y += $(call cc-option,-march=winchip-c6,-march=i586) # Usage: cflags-y += $(call cc-option,-march=winchip-c6,-march=i586)
......
...@@ -4,6 +4,8 @@ OBJECT_FILES_NON_STANDARD := y ...@@ -4,6 +4,8 @@ OBJECT_FILES_NON_STANDARD := y
hostprogs-y := modpost mk_elfconfig hostprogs-y := modpost mk_elfconfig
always := $(hostprogs-y) empty.o always := $(hostprogs-y) empty.o
CFLAGS_REMOVE_empty.o := $(ASM_MACRO_FLAGS)
modpost-objs := modpost.o file2alias.o sumversion.o modpost-objs := modpost.o file2alias.o sumversion.o
devicetable-offsets-file := devicetable-offsets.h devicetable-offsets-file := devicetable-offsets.h
......
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