linux-kernel - [RFC PATCH 2/2] Revert "x86/boot: Add xloadflags bits to check for 5-level paging support"

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Date: Fri,  1 Mar 2024 19:56:18 +0100
From: Borislav Petkov <bp@...en8.de>
To: Baoquan He <bhe@...hat.com>
Cc: X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH 2/2] Revert "x86/boot: Add xloadflags bits to check for 5-level paging support"

From: "Borislav Petkov (AMD)" <bp@...en8.de>

This reverts commit f2d08c5d3bcf3f7ef788af122b57a919efa1e9d0.

This whole dynamic switching support is silly. I don't see a use case
where one would use an old kernel with CONFIG_X86_5LEVEL disabled to
kexec into. I.e., you use pretty much the same kernel.

But I'm open to corrections.

Commit message of

  f2d08c5d3bcf ("x86/boot: Add xloadflags bits to check for 5-level paging support")

claims:

    The flags will be used by the kernel kexec subsystem and the userspace
    kexec tools.

but they're nowhere to be found in kexec tools:

  [ ~/src/kexec-tools> git describe
  v2.0.28-4-g6ee2ac1bf739
  [ ~/src/kexec-tools> git grep XLF_5LEVEL
  [ ~/src/kexec-tools>

Zap it all.

Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
---
 arch/x86/boot/header.S                | 12 +-----------
 arch/x86/include/uapi/asm/bootparam.h |  2 --
 2 files changed, 1 insertion(+), 13 deletions(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index a1bbedd989e4..0f261224acef 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -364,17 +364,7 @@ xloadflags:
 # define XLF4 0
 #endif
 
-#ifdef CONFIG_X86_64
-#ifdef CONFIG_X86_5LEVEL
-#define XLF56 (XLF_5LEVEL|XLF_5LEVEL_ENABLED)
-#else
-#define XLF56 XLF_5LEVEL
-#endif
-#else
-#define XLF56 0
-#endif
-
-			.word XLF0 | XLF1 | XLF23 | XLF4 | XLF56
+			.word XLF0 | XLF1 | XLF23 | XLF4
 
 cmdline_size:   .long   COMMAND_LINE_SIZE-1     #length of the command line,
                                                 #added with boot protocol
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 4a38e7917756..b53b524f6ed2 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -22,8 +22,6 @@
 #define XLF_EFI_HANDOVER_32		(1<<2)
 #define XLF_EFI_HANDOVER_64		(1<<3)
 #define XLF_EFI_KEXEC			(1<<4)
-#define XLF_5LEVEL			(1<<5)
-#define XLF_5LEVEL_ENABLED		(1<<6)
 
 #ifndef __ASSEMBLY__
 
-- 
2.43.0