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

[Kirill A. Shutemov]

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