2022-11-22 16:27:01

by Ard Biesheuvel

Subject: [PATCH v3 09/17] x86/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 <[email protected]>
---
arch/x86/boot/compressed/head_64.S | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 66ad3ab802ca9d0c..f31277f455e63f5f 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -741,7 +741,6 @@ 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
@@ -750,10 +749,8 @@ 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)
@@ -766,7 +763,6 @@ 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)
--
2.35.1

2022-11-24 08:43:51

by tip-bot2 for Alexey Makhalov

Subject: [tip: x86/boot] x86/boot/compressed: Avoid touching ECX in startup32_set_idt_entry()

The following commit has been merged into the x86/boot branch of tip:

Commit-ID: 6aac80a8da46d70f2ae7ff97c9f45a15c7c9b3ef
Gitweb: https://git.kernel.org/tip/6aac80a8da46d70f2ae7ff97c9f45a15c7c9b3ef
Author: Ard Biesheuvel <[email protected]>
AuthorDate: Tue, 22 Nov 2022 17:10:09 +01:00
Committer: Borislav Petkov <[email protected]>
CommitterDate: Thu, 24 Nov 2022 08:57:41 +01:00

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 <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
arch/x86/boot/compressed/head_64.S | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 34d0395..a2d1c03 100644
--- 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)