[PATCHv3 2/5] x86/boot/compressed/64: Print error if 5-level paging is not supported

[Kirill A. Shutemov]

We cannot proceed booting if the machine doesn't support the paging mode
kernel was compiled for.

Getting error the usual way -- via validate_cpu() -- is not going to
work. We need to enable appropriate paging mode before that, otherwise
kernel would triple-fault during KASLR setup.

This code will go away once we get support for boot-time switching
between paging modes.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.14+]
---
 arch/x86/boot/compressed/misc.c       | 16 ++++++++++++++++
 arch/x86/boot/compressed/pgtable_64.c |  2 +-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b50c42455e25..f7f8d9f76e15 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -169,6 +169,16 @@ void __puthex(unsigned long value)
 	}
 }
 
+static int 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)
@@ -362,6 +372,12 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	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;
 
diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index eed3a2c3b577..7bcf03b376da 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -2,7 +2,7 @@
 
 int l5_paging_required(void)
 {
-	/* Check i leaf 7 is supported. */
+	/* Check if leaf 7 is supported. */
 	if (native_cpuid_eax(0) < 7)
 		return 0;
 
-- 
2.15.0