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Subject[PATCHv7 16/19] x86/mm: Allow to boot without la57 if CONFIG_X86_5LEVEL=y
Date
All pieces of the puzzle are in place and we can now allow to boot with
CONFIG_X86_5LEVEL=y on a machine without la57 support.

Kernel will detect that la57 is missing and fold p4d at runtime.

Update documentation and Kconfig option description to reflect the
change.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
Documentation/x86/x86_64/5level-paging.txt | 9 +++------
arch/x86/Kconfig | 4 ++--
arch/x86/include/asm/required-features.h | 8 +-------
3 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/Documentation/x86/x86_64/5level-paging.txt b/Documentation/x86/x86_64/5level-paging.txt
index 087251a0d99c..2432a5ef86d9 100644
--- a/Documentation/x86/x86_64/5level-paging.txt
+++ b/Documentation/x86/x86_64/5level-paging.txt
@@ -20,12 +20,9 @@ Documentation/x86/x86_64/mm.txt

CONFIG_X86_5LEVEL=y enables the feature.

-So far, a kernel compiled with the option enabled will be able to boot
-only on machines that supports the feature -- see for 'la57' flag in
-/proc/cpuinfo.
-
-The plan is to implement boot-time switching between 4- and 5-level paging
-in the future.
+Kernel with CONFIG_X86_5LEVEL=y still able to boot on 4-level hardware.
+In this case additional page table level -- p4d -- will be folded at
+runtime.

== User-space and large virtual address space ==

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f75723d62c25..aaf5c2e00c03 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1411,8 +1411,8 @@ config X86_5LEVEL

It will be supported by future Intel CPUs.

- Note: a kernel with this option enabled can only be booted
- on machines that support the feature.
+ A kernel with the option enabled can be booted on machines that
+ support 4- or 5-level paging.

See Documentation/x86/x86_64/5level-paging.txt for more
information.
diff --git a/arch/x86/include/asm/required-features.h b/arch/x86/include/asm/required-features.h
index d91ba04dd007..fac9a5c0abe9 100644
--- a/arch/x86/include/asm/required-features.h
+++ b/arch/x86/include/asm/required-features.h
@@ -53,12 +53,6 @@
# define NEED_MOVBE 0
#endif

-#ifdef CONFIG_X86_5LEVEL
-# define NEED_LA57 (1<<(X86_FEATURE_LA57 & 31))
-#else
-# define NEED_LA57 0
-#endif
-
#ifdef CONFIG_X86_64
#ifdef CONFIG_PARAVIRT
/* Paravirtualized systems may not have PSE or PGE available */
@@ -104,7 +98,7 @@
#define REQUIRED_MASK13 0
#define REQUIRED_MASK14 0
#define REQUIRED_MASK15 0
-#define REQUIRED_MASK16 (NEED_LA57)
+#define REQUIRED_MASK16 0
#define REQUIRED_MASK17 0
#define REQUIRED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 18)

--
2.14.1
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