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    SubjectRe: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the LA57 check
    On Mon, Mar 12, 2018 at 01:40:27PM +0100, Peter Zijlstra wrote:
    > On Mon, Mar 12, 2018 at 02:27:58AM -0700, tip-bot for Kirill A. Shutemov wrote:
    > > + /*
    > > + * 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;
    > > + }
    >
    > Should this not also include something like: machine actually has
    > suffient memory for it to make sense to use l5 ?

    Define "suffient". :)

    The amount of physical memory is not the only reason to have 5-level
    paging enabled. You may need 5-level paging to get access to wider virtual
    address space to map something not backed by local physical memory
    (consider RDMA).

    --
    Kirill A. Shutemov

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