x86/boot/compressed: Avoid touching ECX in startup32_set_idt_entry()

Avoid touching register %ecx in startup32_set_idt_entry(), by folding
the MOV, SHL and ORL instructions into a single ORL which no longer
requires a temp register.

This permits ECX to be used as a function argument in a subsequent
patch.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-10-ardb@kernel.org

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 34d03953b08c57be9ce599d44171d4e00176ba97..a2d1c031794480c73c3448fd72b5f41e42ed4c5e 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -733,7 +733,6 @@ SYM_DATA_END_LABEL(boot32_idt, SYM_L_GLOBAL, boot32_idt_end)
  */
 SYM_FUNC_START(startup32_set_idt_entry)
        push    %ebx
-       push    %ecx
 
        /* IDT entry address to %ebx */
        leal    rva(boot32_idt)(%ebp), %ebx
@@ -742,10 +741,8 @@ SYM_FUNC_START(startup32_set_idt_entry)
 
        /* Build IDT entry, lower 4 bytes */
        movl    %eax, %edx
-       andl    $0x0000ffff, %edx       # Target code segment offset [15:0]
-       movl    $__KERNEL32_CS, %ecx    # Target code segment selector
-       shl     $16, %ecx
-       orl     %ecx, %edx
+       andl    $0x0000ffff, %edx               # Target code segment offset [15:0]
+       orl     $(__KERNEL32_CS << 16), %edx    # Target code segment selector
 
        /* Store lower 4 bytes to IDT */
        movl    %edx, (%ebx)
@@ -758,7 +755,6 @@ SYM_FUNC_START(startup32_set_idt_entry)
        /* Store upper 4 bytes to IDT */
        movl    %edx, 4(%ebx)
 
-       pop     %ecx
        pop     %ebx
        RET
 SYM_FUNC_END(startup32_set_idt_entry)