Re: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the LA57 check

Peter Zijlstra
2018-03-12T12:40:45Z

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 ?