x86/mm: Allow to boot without LA57 if CONFIG_X86_5LEVEL=y

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 the documentation and the Kconfig option description to reflect the
change.

Signed-off-by: Kirill A. Shutemov <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

Documentation/x86/x86_64/5level-paging.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 ==
 

arch/x86/Kconfig

···
 
 	  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.

arch/x86/boot/compressed/misc.c

···
 	}
 }
 
-static bool l5_supported(void)
-{
-	/* Check if leaf 7 is supported. */
-	if (native_cpuid_eax(0) < 7)
-		return 0;
-
-	/* Check if la57 is supported. */
-	return native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31));
-}
-
 #if CONFIG_X86_NEED_RELOCS
 static void handle_relocations(void *output, unsigned long output_len,
 			       unsigned long virt_addr)
···
 
 	console_init();
 	debug_putstr("early console in extract_kernel\n");
-
-	if (IS_ENABLED(CONFIG_X86_5LEVEL) && !l5_supported()) {
-		error("This linux kernel as configured requires 5-level paging\n"
-			"This CPU does not support the required 'cr4.la57' feature\n"
-			"Unable to boot - please use a kernel appropriate for your CPU\n");
-	}
 
 	free_mem_ptr     = heap;	/* Heap */
 	free_mem_end_ptr = heap + BOOT_HEAP_SIZE;

arch/x86/include/asm/required-features.h

···
 # 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 */
···
 #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_MASK18	0
 #define REQUIRED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 19)