Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
From: liuyuntao (F)
Date: Tue Apr 23 2024 - 08:10:44 EST
On 2024/4/23 8:59, Wei Yang wrote:
Hi, Yuntao
I am not that familiar with this, so got some questions below.
On Mon, Apr 22, 2024 at 06:05:56AM +0000, Yuntao Liu wrote:
The current x86 architecture does not yet support the
HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. x86 is widely used in
embedded scenarios, and enabling this feature would be beneficial for
reducing the size of the kernel image.
In order to make this work, we keep the necessary tables by annotating
them with KEEP, also it requires further changes to linker script to KEEP
some tables and wildcard compiler generated sections into the right place.
Enabling CONFIG_UNWINDER_ORC or CONFIG_MITIGATION_RETPOLINE will enable
the objtool's --orc and --retpoline parameters, which will alter the
layout of the binary file, thereby preventing gc-sections from functioning
properly. Therefore, HAVE_LD_DEAD_CODE_DATA_ELIMINATION should only be
selected when they are not enabled.
Enabling CONFIG_LTO_CLANG or CONFIG_X86_KERNEL_IBT will use vmlinux.o
instead of performing the slow LTO link again. This can also prevent
gc-sections from functioning properly. Therefore, using this optimization
when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not enabled.
The size comparison of zImage is as follows:
x86_def_defconfig i386_defconfig tinyconfig
10892288 10826240 607232 no dce
10748928 10719744 529408 dce
1.3% 0.98% 12.8% shrink
When using smaller config file, there is a significant reduction in the
size of the zImage.
---
arch/x86/Kconfig | 1 +
arch/x86/kernel/vmlinux.lds.S | 24 ++++++++++++------------
scripts/link-vmlinux.sh | 2 +-
3 files changed, 14 insertions(+), 13 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index a902680b6537..92dfbc8ee4e7 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -247,6 +247,7 @@ config X86
select HAVE_FUNCTION_ERROR_INJECTION
select HAVE_KRETPROBES
select HAVE_RETHOOK
+ select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !CONFIG_UNWINDER_ORC && !CONFIG_MITIGATION_RETPOLINE
How to make sure only these two config affect the function?
I have tested all the parameters of objtool, and only when these two
config are removed can gc-sections take effect.
select HAVE_LIVEPATCH if X86_64
select HAVE_MIXED_BREAKPOINTS_REGS
select HAVE_MOD_ARCH_SPECIFIC
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 3509afc6a672..aeee2b9b6a6a 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -253,7 +253,7 @@ SECTIONS
.x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) {
__x86_cpu_dev_start = .;
- *(.x86_cpu_dev.init)
+ KEEP(*(.x86_cpu_dev.init))
How you choose the sections to keep?
If my understanding is correct, keep the sections is for gc-section. Sounds we
need to keep all possible section, otherwise it would mis-behave?
Nicholas Piggin has mentioned it before.
FYI, easiest way to check if you forgot to KEEP a linker table is
to look at `readelf -S vmlinux` differences, and to see what is
being trimmed, look at nm differences or use --print-gc-sections
LD option to see what symbols you're trimming. Linker tables,
boot entry, and exception entry tends to require anchoring.
https://lore.kernel.org/all/20170709031333.29443-1-npiggin@xxxxxxxxx/
__x86_cpu_dev_end = .;
}
@@ -261,7 +261,7 @@ SECTIONS
.x86_intel_mid_dev.init : AT(ADDR(.x86_intel_mid_dev.init) - \
LOAD_OFFSET) {
__x86_intel_mid_dev_start = .;
- *(.x86_intel_mid_dev.init)
+ KEEP(*(.x86_intel_mid_dev.init))
__x86_intel_mid_dev_end = .;
}
#endif
@@ -275,21 +275,21 @@ SECTIONS
. = ALIGN(8);
.retpoline_sites : AT(ADDR(.retpoline_sites) - LOAD_OFFSET) {
__retpoline_sites = .;
- *(.retpoline_sites)
+ KEEP(*(.retpoline_sites))
__retpoline_sites_end = .;
}
. = ALIGN(8);
.return_sites : AT(ADDR(.return_sites) - LOAD_OFFSET) {
__return_sites = .;
- *(.return_sites)
+ KEEP(*(.return_sites))
__return_sites_end = .;
}
. = ALIGN(8);
.call_sites : AT(ADDR(.call_sites) - LOAD_OFFSET) {
__call_sites = .;
- *(.call_sites)
+ KEEP(*(.call_sites))
__call_sites_end = .;
}
#endif
@@ -298,7 +298,7 @@ SECTIONS
. = ALIGN(8);
.ibt_endbr_seal : AT(ADDR(.ibt_endbr_seal) - LOAD_OFFSET) {
__ibt_endbr_seal = .;
- *(.ibt_endbr_seal)
+ KEEP(*(.ibt_endbr_seal))
__ibt_endbr_seal_end = .;
}
#endif
@@ -307,7 +307,7 @@ SECTIONS
. = ALIGN(8);
.cfi_sites : AT(ADDR(.cfi_sites) - LOAD_OFFSET) {
__cfi_sites = .;
- *(.cfi_sites)
+ KEEP(*(.cfi_sites))
__cfi_sites_end = .;
}
#endif
@@ -320,7 +320,7 @@ SECTIONS
. = ALIGN(8);
.altinstructions : AT(ADDR(.altinstructions) - LOAD_OFFSET) {
__alt_instructions = .;
- *(.altinstructions)
+ KEEP(*(.altinstructions))
__alt_instructions_end = .;
}
@@ -330,13 +330,13 @@ SECTIONS
* get the address and the length of them to patch the kernel safely.
*/
.altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) {
- *(.altinstr_replacement)
+ KEEP(*(.altinstr_replacement))
}
. = ALIGN(8);
.apicdrivers : AT(ADDR(.apicdrivers) - LOAD_OFFSET) {
__apicdrivers = .;
- *(.apicdrivers);
+ KEEP(*(.apicdrivers));
__apicdrivers_end = .;
}
@@ -406,7 +406,7 @@ SECTIONS
.brk : AT(ADDR(.brk) - LOAD_OFFSET) {
__brk_base = .;
. += 64 * 1024; /* 64k alignment slop space */
- *(.bss..brk) /* areas brk users have reserved */
+ KEEP(*(.bss..brk)) /* areas brk users have reserved */
__brk_limit = .;
}
@@ -432,7 +432,7 @@ SECTIONS
. = ALIGN(HPAGE_SIZE);
.init.scratch : AT(ADDR(.init.scratch) - LOAD_OFFSET) {
__init_scratch_begin = .;
- *(.init.scratch)
+ KEEP(*(.init.scratch))
. = ALIGN(HPAGE_SIZE);
__init_scratch_end = .;
}
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 7862a8101747..7287b5a9f17d 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -60,7 +60,7 @@ vmlinux_link()
# skip output file argument
shift
- if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
+ if [ is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT ] && [ ! is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION ]; then
# Use vmlinux.o instead of performing the slow LTO link again.
objs=vmlinux.o
libs=
--
2.34.1