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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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