x86/boot/compressed: Simplify IDT/GDT preserve/restore in the EFI thunk

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@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221122161017.2426828-9-ardb@kernel.org

arch/x86/boot/compressed/efi_mixed.S

@@ -96,24 +96,20 @@ 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 by the efi32_entry() routine 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 by the efi32_entry() routine 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
@@ -187,9 +183,7 @@ SYM_FUNC_START_LOCAL(efi_enter32)
 	 */
 	cli
 
-	lidtl	(%ebx)
-	subl	$16, %ebx
-
+	lidtl	16(%ebx)
 	lgdtl	(%ebx)
 
 	movl	%cr4, %eax