x86_64: Re-positioning the bss segment
PATCH 1/1 x86_64: Re-positioning the bss segment
o Currently bss segment is being placed somewhere in the middle (after .data) section and after bss lots of init section and data sections are coming. Is it intentional?
o One side affect of placing bss in the middle is that objcopy keeps the bss in raw binary image (vmlinux.bin) hence unnecessarily increasing the size of raw binary image. (In my case ~600K). It also increases the size of generated bzImage, though the increase is very small (896 bytes), probably a very high compression ratio for stream of zeros.
o This patch moves the bss at the end hence reducing the size of bzImage by 896 bytes and size of vmlinux.bin by 600K.
o This change benefits in the context of relocatable kernel patches. If kernel bss is not part of compressed data (vmlinux.bin) then it does not have to be decompressed and this area can be used by the decompressor for its execution hence keeping the memory requirements bounded and decompressor code does not stomp over any other data loaded beyond kernel image (As might be the case with bootloaders like kexec).
---
arch/x86_64/kernel/vmlinux.lds.S | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-)
arch/x86_64/kernel/vmlinux.lds.S renamed from arch/x86_64/kernel/vmlinux.lds.S~x86_64-bss-segment-placing-fix +7 -7
@@ -61,13 +61,6 @@ SECTIONS _edata = .; /* End of data section */ - __bss_start = .; /* BSS */ - .bss : AT(ADDR(.bss) - LOAD_OFFSET) { - *(.bss.page_aligned) - *(.bss) - } - __bss_stop = .; - . = ALIGN(PAGE_SIZE); . = ALIGN(CONFIG_X86_L1_CACHE_BYTES); .data.cacheline_aligned : AT(ADDR(.data.cacheline_aligned) - LOAD_OFFSET) {
@@ -222,6 +215,13 @@ SECTIONS . = ALIGN(4096); __nosave_end = .; + __bss_start = .; /* BSS */ + .bss : AT(ADDR(.bss) - LOAD_OFFSET) { + *(.bss.page_aligned) + *(.bss) + } + __bss_stop = .; + _end = . ; /* Sections to be discarded */
_
Quoted 14 lines · Vivek Goyal
o Currently bss segment is being placed somewhere in the middle (after .data) section and after bss lots of init section and data sections are coming. Is it intentional?
o One side affect of placing bss in the middle is that objcopy keeps the bss in raw binary image (vmlinux.bin) hence unnecessarily increasing the size of raw binary image. (In my case ~600K). It also increases the size of generated bzImage, though the increase is very small (896 bytes), probably a very high compression ratio for stream of zeros.
o This patch moves the bss at the end hence reducing the size of bzImage by 896 bytes and size of vmlinux.bin by 600K.
Quoted 6 lines
o This change benefits in the context of relocatable kernel patches. If kernel bss is not part of compressed data (vmlinux.bin) then it does not have to be decompressed and this area can be used by the decompressor for its execution hence keeping the memory requirements bounded and decompressor code does not stomp over any other data loaded beyond kernel image (As might be the case with bootloaders like kexec).
Looks sane here. I don't know if there are any side effects from placing the .bss after the .init sections that we discard at after boot.
Unless this has undesirable side effects this should have us doing the expected thing, with the .bss section which is less likely to confuse people in general.
Quoted 1 line
Signed-off-by Vivek Goyal <[email protected]>
Eric
Quoted 4 lines · On Tue, 15 Aug 2006 17:49:52 -0400 Vivek Goyal <[email protected]> wrote:
o Currently bss segment is being placed somewhere in the middle (after .data) section and after bss lots of init section and data sections are coming. Is it intentional?
Not that I know of.
Quoted 11 lines
o One side affect of placing bss in the middle is that objcopy keeps the bss in raw binary image (vmlinux.bin) hence unnecessarily increasing the size of raw binary image. (In my case ~600K). It also increases the size of generated bzImage, though the increase is very small (896 bytes), probably a very high compression ratio for stream of zeros.
o This patch moves the bss at the end hence reducing the size of bzImage by 896 bytes and size of vmlinux.bin by 600K.
Quoted 6 lines
o This change benefits in the context of relocatable kernel patches. If kernel bss is not part of compressed data (vmlinux.bin) then it does not have to be decompressed and this area can be used by the decompressor for its execution hence keeping the memory requirements bounded and decompressor code does not stomp over any other data loaded beyond kernel image (As might be the case with bootloaders like kexec).
Merged thanks.
Does i386 need a similar change?
-Andi
Quoted 28 lines · Andi Kleen
Quoted 4 lines · On Tue, 15 Aug 2006 17:49:52 -0400 Vivek Goyal <[email protected]> wrote:
o Currently bss segment is being placed somewhere in the middle (after .data) section and after bss lots of init section and data sections are coming. Is it intentional?
Not that I know of.
Quoted 11 lines
o One side affect of placing bss in the middle is that objcopy keeps the bss in raw binary image (vmlinux.bin) hence unnecessarily increasing the size of raw binary image. (In my case ~600K). It also increases the size of generated bzImage, though the increase is very small (896 bytes), probably a very high compression ratio for stream of zeros.
o This patch moves the bss at the end hence reducing the size of bzImage by 896 bytes and size of vmlinux.bin by 600K.
Quoted 6 lines
o This change benefits in the context of relocatable kernel patches. If kernel bss is not part of compressed data (vmlinux.bin) then it does not have to be decompressed and this area can be used by the decompressor for its execution hence keeping the memory requirements bounded and decompressor code does not stomp over any other data loaded beyond kernel image (As might be the case with bootloaders like kexec).
Quoted 3 lines
Merged thanks.
Does i386 need a similar change?
Nope. i386 already has bss at the end.
Thanks
Vivek
Quoted 11 lines · Andi Kleen
Quoted 10 lines
o This patch moves the bss at the end hence reducing the size of bzImage by 896 bytes and size of vmlinux.bin by 600K.
o This change benefits in the context of relocatable kernel patches. If kernel bss is not part of compressed data (vmlinux.bin) then it does not have to be decompressed and this area can be used by the decompressor for its execution hence keeping the memory requirements bounded and decompressor code does not stomp over any other data loaded beyond kernel image (As might be the case with bootloaders like kexec).
Quoted 3 lines
Merged thanks.
Does i386 need a similar change?
If it does then I suggest moving the BSS definition to include/asm-generic/vmlinux.lds.h
Sam