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

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kirill.shutemov@linux.intel.com, gorcunov@openvz.org,
	luto@amacapital.net, keescook@chromium.org, willy@infradead.org,
	torvalds@linux-foundation.org, tglx@linutronix.de, bp@suse.de,
	andy.shevchenko@gmail.com, linux-kernel@vger.kernel.org,
	hpa@zytor.com, mingo@kernel.org, ebiederm@xmission.com,
	jgross@suse.com, linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the LA57 check
Date: Mon, 12 Mar 2018 15:43:37 +0300	[thread overview]
Message-ID: <20180312124337.vw7bchm6brfzghfa@node.shutemov.name> (raw)
In-Reply-To: <20180312124027.GG4064@hirez.programming.kicks-ass.net>

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