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

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Message-ID: <20180312124337.vw7bchm6brfzghfa@node.shutemov.name>
Date:   Mon, 12 Mar 2018 15:43:37 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kirill.shutemov@...ux.intel.com, gorcunov@...nvz.org,
        luto@...capital.net, keescook@...omium.org, willy@...radead.org,
        torvalds@...ux-foundation.org, tglx@...utronix.de, bp@...e.de,
        andy.shevchenko@...il.com, linux-kernel@...r.kernel.org,
        hpa@...or.com, mingo@...nel.org, ebiederm@...ssion.com,
        jgross@...e.com, linux-tip-commits@...r.kernel.org
Subject: Re: [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