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

The patch explains the LA57 check in more details.

Tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180226180451.86788-2-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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;
 }