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x86/boot: Micro-optimize GDT loading instructions

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Rearrange the instructions a bit to use a 32-bit displacement once
instead of 2/3 times. This saves 8 bytes of machine code.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-8-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
This commit is contained in:
Arvind Sankar 2020-02-02 12:13:53 -05:00 committed by Ard Biesheuvel
parent b75e2b076d
commit 8a3abe30de
1 changed files with 8 additions and 7 deletions
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15
arch/x86/boot/compressed/head_64.S
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@ -69,8 +69,9 @@ SYM_FUNC_START(startup_32)
subl $1b, %ebp
/* Load new GDT with the 64bit segments using 32bit descriptor */
addl %ebp, gdt+2(%ebp)
lgdt gdt(%ebp)
leal gdt(%ebp), %eax
movl %eax, 2(%eax)
lgdt (%eax)
/* Load segment registers with our descriptors */
movl $__BOOT_DS, %eax
@ -355,9 +356,9 @@ SYM_CODE_START(startup_64)
*/
/* Make sure we have GDT with 32-bit code segment */
leaq gdt(%rip), %rax
movq %rax, gdt64+2(%rip)
lgdt gdt64(%rip)
leaq gdt64(%rip), %rax
addq %rax, 2(%rax)
lgdt (%rax)
/*
* paging_prepare() sets up the trampoline and checks if we need to
@ -625,12 +626,12 @@ SYM_FUNC_END(.Lno_longmode)
.data
SYM_DATA_START_LOCAL(gdt64)
.word gdt_end - gdt - 1
.quad 0
.quad gdt - gdt64
SYM_DATA_END(gdt64)
.balign 8
SYM_DATA_START_LOCAL(gdt)
.word gdt_end - gdt - 1
.long gdt
.long 0
.word 0
.quad 0x00cf9a000000ffff /* __KERNEL32_CS */
.quad 0x00af9a000000ffff /* __KERNEL_CS */