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Message-ID: <2397d8dd-8053-4167-9aac-f5d7c05a98da@huawei.com>
Date: Tue, 27 Feb 2024 16:06:56 +0800
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Subject: Re: [PATCH -next] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
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To: Arnd Bergmann <arnd@arndb.de>, <linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>
CC: Russell King <linux@armlinux.org.uk>, Andrew Davis <afd@ti.com>, Andrew
 Morton <akpm@linux-foundation.org>, "Kirill A. Shutemov"
	<kirill.shutemov@linux.intel.com>, Geert Uytterhoeven
	<geert+renesas@glider.be>, Jonathan Corbet <corbet@lwn.net>, Mike Rapoport
	<rppt@kernel.org>, Rob Herring <robh@kernel.org>, Thomas Gleixner
	<tglx@linutronix.de>, Linus Walleij <linus.walleij@linaro.org>
References: <20240220081527.23408-1-liuyuntao12@huawei.com>
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 <38c09a4b-69cc-4dc5-8a68-e5f5597613ac@huawei.com>
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 <4e9396ca-460b-49ca-818e-73f0a8997b15@huawei.com>
 <58297ee9-4f33-4b3b-bd00-b44e86965892@app.fastmail.com>
From: "liuyuntao (F)" <liuyuntao12@huawei.com>
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On 2024/2/23 0:04, Arnd Bergmann wrote:
> On Thu, Feb 22, 2024, at 12:24, liuyuntao (F) wrote:
>>
>> The position of the caret has been moved below the right brace
>> of { KEEP(*(.vectors.bhb.loop8)) }, indicating that lld is treating
>> the entire `KEEP(*(.vectors))` as a file name. This could potentially be
>> a bug in lld. Perhaps we can temporarily
>> enable the DCE option only when option LD_IS_LLD is disabled,
>> like risc-v:
>>
>> `select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD`.
> 
> I would really like to see this working with lld if at all
> possible, as it allows the combination of gc-sections with
> lto and CONFIG_TRIM_UNUSED_KSYMS.
> 
> I experimented with lld myself now and I did get a booting
> kernel even without the the KEEP() on the vectors. I also
> see that this is the only use of OVERLAY in the kernel, so
> I hope that we can find a way to make it work with existing
> lld after all, either without the KEEP or without the OVERLAY.
> 
> Did you see any problems without the KEEP() on the vectors?
> 
>       Arnd

Hi, Arnd. I have added a global symbol g_keep1 in .vectors, g_keep2 in 
vectors.bhb.loop8 and g_keep3 in .vectors.bhb.bpiall respectively. I 
also added another section to reference these three global symbols, to 
prevent these sections from being removed during linking with --gc-sections.

It worked,but there should be a better way to achieve it. the patch:

diff --git a/arch/arm/include/asm/vmlinux.lds.h 
b/arch/arm/include/asm/vmlinux.lds.h
index f2ff79f740ab..d60f6e83a9f7 100644
--- a/arch/arm/include/asm/vmlinux.lds.h
+++ b/arch/arm/include/asm/vmlinux.lds.h
@@ -125,13 +125,13 @@
         __vectors_lma = .;                                              \
         OVERLAY 0xffff0000 : NOCROSSREFS AT(__vectors_lma) {            \
                 .vectors {                                              \
-                       KEEP(*(.vectors))                               \
+                       *(.vectors)                                     \
                 }                                                       \
                 .vectors.bhb.loop8 {                                    \
-                       KEEP(*(.vectors.bhb.loop8))                     \
+                       *(.vectors.bhb.loop8)                           \
                 }                                                       \
                 .vectors.bhb.bpiall {                                   \
-                       KEEP(*(.vectors.bhb.bpiall))                    \
+                       *(.vectors.bhb.bpiall)                          \
                 }                                                       \
         }                                                               \
         ARM_LMA(__vectors, .vectors);                                   \
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 6150a716828c..84536e805da0 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -1075,6 +1075,9 @@ THUMB(    .reloc  ., R_ARM_THM_PC12, 
L__vector_swi               )
         W(b)    vector_addrexcptn
         W(b)    vector_irq
         W(b)    vector_fiq
+       .global g_keep1
+       g_keep1:
+               .word 0

  #ifdef CONFIG_HARDEN_BRANCH_HISTORY
         .section .vectors.bhb.loop8, "ax", %progbits
@@ -1088,6 +1091,9 @@ THUMB(    .reloc  ., R_ARM_THM_PC12, 
L__vector_bhb_loop8_swi     )
         W(b)    vector_addrexcptn
         W(b)    vector_bhb_loop8_irq
         W(b)    vector_bhb_loop8_fiq
+       .global g_keep2
+       g_keep2:
+               .word 0

         .section .vectors.bhb.bpiall, "ax", %progbits
         W(b)    vector_rst
@@ -1100,6 +1106,9 @@ THUMB(    .reloc  ., R_ARM_THM_PC12, 
L__vector_bhb_bpiall_swi    )
         W(b)    vector_addrexcptn
         W(b)    vector_bhb_bpiall_irq
         W(b)    vector_bhb_bpiall_fiq
+       .global g_keep3
+       g_keep3:
+               .word 0
  #endif

         .data
@@ -1108,3 +1117,8 @@ THUMB(    .reloc  ., R_ARM_THM_PC12, 
L__vector_bhb_bpiall_swi    )
         .globl  cr_alignment
  cr_alignment:
         .space  4
+
+.section .keep_vectors, "ax", %progbits
+       LDR r0, =g_keep1
+       LDR r1, =g_keep2
+       LDR r2, =g_keep3
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 01a887c1141a..5cdfb4ba3ac4 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -62,6 +62,7 @@ SECTIONS
         .text : {                       /* Real text segment            */
                 _stext = .;             /* Text and read-only data      */
                 ARM_TEXT
+               KEEP(*(.keep_vectors))
         }

  #ifdef CONFIG_DEBUG_ALIGN_RODATA
-- 
2.34.1

 > I would really like to see this working with lld if at all
 > possible, as it allows the combination of gc-sections with
 > lto and CONFIG_TRIM_UNUSED_KSYMS.
Then, I enabled config CONFIG_LTO_CLANG_THIN in arm32, but
came across a link failure using clang/lld:

following symbols must have non local/private scope:
free_mem_end_ptr
free_mem_ptr
malloc_count
malloc_ptr
output_data

in file arch/arm/boot/compressed/Makefile:
# We need to prevent any GOTOFF relocs being used with references
# to symbols in the .bss section since we cannot relocate them
# independently from the rest at run time.  This can be achieved by
# ensuring that no private .bss symbols exist, as global symbols
# always have a GOT entry which is what we need.
# The .data section is already discarded by the linker script so no need
# to bother about it here.
check_for_bad_syms = \
bad_syms=$$($(NM) $@ | sed -n 's/^.\{8\} [bc] \(.*\)/\1/p') && \
[ -z "$$bad_syms" ] || \
   ( echo "following symbols must have non local/private scope:" >&2; \
     echo "$$bad_syms" >&2; false )

Turn on the config CONFIG_LTO_CLANG_THIN , use nm to check the type of 
these symbols as 'b', turn off the config type to 'B'. I tried to 
explicitly declare these variables using 
__attribute__((visibility("default")), but it didn't take effect on the 
variable `output_data`.
ld.lld: warning: 
thinlto-cache/llvmcache-285C4672B80361F1DA67C743A5C8350FDDEEC8E3:(.data.malloc_ptr) 
is being placed in '.data.':

ld.lld: warning: 
thinlto-cache/llvmcache-285C4672B80361F1DA67C743A5C8350FDDEEC8E3:(.data.malloc_ptr) 
is being placed in '.data.'
ld.lld: warning: 
thinlto-cache/llvmcache-285C4672B80361F1DA67C743A5C8350FDDEEC8E3:(.data.malloc_count) 
is being placed in '.dat'
following symbols must have non local/private scope:
output_data