Commit 8a3abe30 authored by Arvind Sankar's avatar Arvind Sankar Committed by Ard Biesheuvel
Browse files

x86/boot: Micro-optimize GDT loading instructions 



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: default avatarArvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-8-nivedita@alum.mit.edu


Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
parent b75e2b07

arch/x86/boot/compressed/head_64.S

+8 −7
Original line number Diff line number Diff line
@@ -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 */

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