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: 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>

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index c36e6156b6a31c1dd151ff2caf9ed284e87ac4d0..a4f5561c1c0eda3b1665cb2fb7f2fc415c27ede9 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -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 */