linux-kernel - [RFC PATCH 3/9] x86/purgatory: Drop pointless GDT switch

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Date: Wed, 24 Apr 2024 17:53:13 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>
Subject: [RFC PATCH 3/9] x86/purgatory: Drop pointless GDT switch

From: Ard Biesheuvel <ardb@...nel.org>

The x86 purgatory switches to a new GDT twice, and the first time, it
doesn't even bother to switch to the new code segment. Given that data
segment selectors are ignored in long mode, and the fact that the GDT is
reprogrammed again after returning from purgatory(), the first switch is
entirely pointless and can just be dropped altogether.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/purgatory/setup-x86_64.S | 29 --------------------
 1 file changed, 29 deletions(-)

diff --git a/arch/x86/purgatory/setup-x86_64.S b/arch/x86/purgatory/setup-x86_64.S
index 2d10ff88851d..f160fc729cbe 100644
--- a/arch/x86/purgatory/setup-x86_64.S
+++ b/arch/x86/purgatory/setup-x86_64.S
@@ -15,17 +15,6 @@
 	.code64
 
 SYM_CODE_START(purgatory_start)
-	/* Load a gdt so I know what the segment registers are */
-	lgdt	gdt(%rip)
-
-	/* load the data segments */
-	movl	$0x18, %eax	/* data segment */
-	movl	%eax, %ds
-	movl	%eax, %es
-	movl	%eax, %ss
-	movl	%eax, %fs
-	movl	%eax, %gs
-
 	/* Setup a stack */
 	leaq	lstack_end(%rip), %rsp
 
@@ -34,24 +23,6 @@ SYM_CODE_START(purgatory_start)
 	jmp	entry64
 SYM_CODE_END(purgatory_start)
 
-	.section ".rodata"
-	.balign 16
-SYM_DATA_START_LOCAL(gdt)
-	/* 0x00 unusable segment
-	 * 0x08 unused
-	 * so use them as the gdt ptr
-	 */
-	.word	gdt_end - gdt - 1
-	.quad	gdt
-	.word	0, 0, 0
-
-	/* 0x10 4GB flat code segment */
-	.word	0xFFFF, 0x0000, 0x9A00, 0x00AF
-
-	/* 0x18 4GB flat data segment */
-	.word	0xFFFF, 0x0000, 0x9200, 0x00CF
-SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)
-
 	.bss
 	.balign 16
 SYM_DATA_START_LOCAL(lstack)
-- 
2.44.0.769.g3c40516874-goog