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Subject[tip:x86/boot] x86/boot: Add xloadflags bits to check for 5-level paging support
Commit-ID:  f2d08c5d3bcf3f7ef788af122b57a919efa1e9d0
Author: Baoquan He <>
AuthorDate: Fri, 24 May 2019 15:38:08 +0800
Committer: Thomas Gleixner <>
CommitDate: Fri, 28 Jun 2019 07:14:59 +0200

x86/boot: Add xloadflags bits to check for 5-level paging support

The current kernel supports 5-level paging mode, and supports dynamically
choosing the paging mode during bootup depending on the kernel image,
hardware and kernel parameter settings. This flexibility brings several
issues to kexec/kdump:

1) Dynamic switching between paging modes requires support in the target
kernel. This means kexec from a 5-level paging kernel into a kernel
which does not support mode switching is not possible. So the loader
needs to be able to analyze the supported paging modes of the kexec
target kernel.

2) If running on a 5-level paging kernel and the kexec target kernel is a
4-level paging kernel, the target immage cannot be loaded above the 64TB
address space limit. But the kexec loader searches for a load area from
top to bottom which would eventually put the target kernel above 64TB
when the machine has large enough RAM size. So the loader needs to be
able to analyze the paging mode of the target kernel to load it at a
suitable spot in the address space.



- Bit XLF_5LEVEL indicates whether 5-level paging mode switching support
is available. (Issue #1)

- Bit XLF_5LEVEL_ENABLED indicates whether the kernel was compiled with
full 5-level paging support (CONFIG_X86_5LEVEL=y). (Issue #2)

The loader will use these bits to verify whether the target kernel is
suitable to be kexec'ed to from a 5-level paging kernel and to determine
the constraints of the target kernel load address.

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

[ tglx: Massaged changelog ]

Signed-off-by: Baoquan He <>
Signed-off-by: Thomas Gleixner <>
Acked-by: Kirill A. Shutemov <>

arch/x86/boot/header.S | 12 +++++++++++-
arch/x86/include/uapi/asm/bootparam.h | 2 ++
2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 850b8762e889..be19f4199727 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -419,7 +419,17 @@ xloadflags:
# define XLF4 0

- .word XLF0 | XLF1 | XLF23 | XLF4
+#ifdef CONFIG_X86_64
+#ifdef CONFIG_X86_5LEVEL
+#define XLF56 XLF_5LEVEL
+#define XLF56 0
+ .word XLF0 | XLF1 | XLF23 | XLF4 | XLF56

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 60733f137e9a..c895df5482c5 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -29,6 +29,8 @@
#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__

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