linux-kernel - [PATCHv2 1/5] x86/boot/compressed/64: Describe the logic behind LA57 check

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Date:   Tue, 27 Feb 2018 18:42:13 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, Andi Kleen <ak@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv2 1/5] x86/boot/compressed/64: Describe the logic behind LA57 check

The patch explains the LA57 check in more details.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 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;
 }
-- 
2.16.1