x86/boot/compressed/64: Describe the logic behind the LA57 check

The patch explains the LA57 check in more details.

Tested-by: Borislav Petkov <[email protected]>
Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 15 insertions, 3 deletions

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index 3f1697fcc7a8..45c76eff2718 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -18,10 +18,22 @@ struct paging_config paging_prepare(void)
 {
 	struct paging_config paging_config = {};
 
-	/* Check if LA57 is desired and supported */
-	if (IS_ENABLED(CONFIG_X86_5LEVEL) && native_cpuid_eax(0) >= 7 &&
-			(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31))))
+	/*
+	 * Check if LA57 is desired and supported.
+	 *
+	 * There are two parts to the check:
+	 *   - if the kernel supports 5-level paging: CONFIG_X86_5LEVEL=y
+	 *   - if the machine supports 5-level paging:
+	 *     + CPUID leaf 7 is supported
+	 *     + the leaf has the feature bit set
+	 *
+	 * That's substitute for boot_cpu_has() in early boot code.
+	 */
+	if (IS_ENABLED(CONFIG_X86_5LEVEL) &&
+			native_cpuid_eax(0) >= 7 &&
+			(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31)))) {
 		paging_config.l5_required = 1;
+	}
 
 	return paging_config;
 }