linux-kernel - [PATCH 2/6] efi/x86: simplify IDT/GDT preserve/restore

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Message-Id: <20220815134223.740112-3-ardb@kernel.org>
Date:   Mon, 15 Aug 2022 15:42:19 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-efi@...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 2/6] efi/x86: simplify IDT/GDT preserve/restore

Tweak the asm and remove some redundant instructions. While at it,
fix the associated comment for style and correctness.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/boot/compressed/efi_mixed.S | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
index 6fd7ac517c53..a0f217c8ab38 100644
--- a/arch/x86/boot/compressed/efi_mixed.S
+++ b/arch/x86/boot/compressed/efi_mixed.S
@@ -94,24 +94,20 @@ SYM_FUNC_START(__efi64_thunk)
 
 	leaq	0x20(%rsp), %rbx
 	sgdt	(%rbx)
-
-	addq	$16, %rbx
-	sidt	(%rbx)
+	sidt	16(%rbx)
 
 	leaq	1f(%rip), %rbp
 
 	/*
-	 * Switch to IDT and GDT with 32-bit segments. This is the firmware GDT
-	 * and IDT that was installed when the kernel started executing. The
-	 * pointers were saved at the efi32_stub_handover entry point below.
+	 * Switch to IDT and GDT with 32-bit segments. These are the firmware
+	 * GDT and IDT that were installed when the kernel started executing.
+	 * The pointers were saved at the efi32_entry entry point below.
 	 *
 	 * Pass the saved DS selector to the 32-bit code, and use far return to
 	 * restore the saved CS selector.
 	 */
-	leaq	efi32_boot_idt(%rip), %rax
-	lidt	(%rax)
-	leaq	efi32_boot_gdt(%rip), %rax
-	lgdt	(%rax)
+	lidt	efi32_boot_idt(%rip)
+	lgdt	efi32_boot_gdt(%rip)
 
 	movzwl	efi32_boot_ds(%rip), %edx
 	movzwq	efi32_boot_cs(%rip), %rax
@@ -185,9 +181,7 @@ SYM_FUNC_START_LOCAL(efi_enter32)
 	 */
 	cli
 
-	lidtl	(%ebx)
-	subl	$16, %ebx
-
+	lidtl	16(%ebx)
 	lgdtl	(%ebx)
 
 	movl	%cr4, %eax
-- 
2.35.1