Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Wei Yang]

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?

> 	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?

> 		__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

-- 
Wei Yang
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