[PATCH 6.1 118/215] x86/boot/compressed: Avoid touching ECX in startup32_set_idt_entry()

Greg Kroah-Hartman
2024-03-04T21:47:24Z

From: Ard Biesheuvel <ardb@kernel.org>

commit 6aac80a8da46d70f2ae7ff97c9f45a15c7c9b3ef upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/boot/compressed/head_64.S | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)

--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -749,7 +749,6 @@ SYM_DATA_END_LABEL(boot32_idt, SYM_L_GLO
*/
SYM_FUNC_START(startup32_set_idt_entry)
push %ebx
- push %ecx

/* IDT entry address to %ebx */
leal rva(boot32_idt)(%ebp), %ebx
@@ -758,10 +757,8 @@ 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)
@@ -774,7 +771,6 @@ 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)