linux-kernel - [PATCH v2 09/16] x86/compressed: avoid touching ECX in startup32_set_idt

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Date:   Wed, 21 Sep 2022 16:54:15 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-efi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michael Roth <michael.roth@....com>
Subject: [PATCH v2 09/16] 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 <ardb@...nel.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 90b119fbef58..3db7e4a634b0 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)
-- 
2.35.1