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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@linux.intel.com>
AuthorDate: Mon, 26 Feb 2018 21:04:47 +0300
Committer: Ingo Molnar <mingo@kernel.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@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 +++++++++++++++---
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;
}
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