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Date:   Thu, 24 Nov 2022 08:12:07 -0000
From:   "tip-bot2 for Ard Biesheuvel" <tip-bot2@linutronix.de>
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To:     linux-tip-commits@vger.kernel.org
Subject: [tip: x86/boot] x86/boot/compressed: Avoid touching ECX in
 startup32_set_idt_entry()
Cc:     Ard Biesheuvel <ardb@kernel.org>, Borislav Petkov <bp@suse.de>,
        x86@kernel.org, linux-kernel@vger.kernel.org
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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 <ardb@kernel.org>
AuthorDate:    Tue, 22 Nov 2022 17:10:09 +01:00
Committer:     Borislav Petkov <bp@suse.de>
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 <ardb@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-10-ardb@kernel.org
---
 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)