linux-kernel - [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the LA57 check

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Message-ID: <tip-a403d798182f4f7be5e9bab56cfa37e9828fd92a@git.kernel.org>
Date:   Mon, 12 Mar 2018 02:27:58 -0700
From:   "tip-bot for Kirill A. Shutemov" <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     kirill.shutemov@...ux.intel.com, gorcunov@...nvz.org,
        keescook@...omium.org, luto@...capital.net, willy@...radead.org,
        torvalds@...ux-foundation.org, andy.shevchenko@...il.com,
        bp@...e.de, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        mingo@...nel.org, hpa@...or.com, ebiederm@...ssion.com,
        peterz@...radead.org, jgross@...e.com
Subject: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the
 LA57 check

Commit-ID:  a403d798182f4f7be5e9bab56cfa37e9828fd92a
Gitweb:     https://git.kernel.org/tip/a403d798182f4f7be5e9bab56cfa37e9828fd92a
Author:     Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
AuthorDate: Mon, 26 Feb 2018 21:04:47 +0300
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 12 Mar 2018 09:29:24 +0100

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@...e.de>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
Cc: Eric Biederman <ebiederm@...ssion.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-mm@...ck.org
Link: http://lkml.kernel.org/r/20180226180451.86788-2-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/boot/compressed/pgtable_64.c | 18 +++++++++++++++---
 1 file 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;
 }