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Subject[PATCH 1/5] x86/boot/compressed/64: Describe the logic behind LA57 check
Date
The patch explains the LA57 check in more details.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.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
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