linux-kernel - Re: [PATCH v4 4/7] arm64: Add a description on 48-bit address space with 4KB pages

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Date:	Wed, 30 Apr 2014 15:41:40 +0900
From:	Jungseok Lee <jays.lee@...sung.com>
To:	'Catalin Marinas' <catalin.marinas@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
	'Marc Zyngier' <Marc.Zyngier@....com>,
	'Christoffer Dall' <christoffer.dall@...aro.org>,
	linux-kernel@...r.kernel.org,
	'linux-samsung-soc' <linux-samsung-soc@...r.kernel.org>,
	steve.capper@...aro.org, sungjinn.chung@...sung.com,
	'Arnd Bergmann' <arnd@...db.de>, kgene.kim@...sung.com,
	ilho215.lee@...sung.com
Subject: Re: [PATCH v4 4/7] arm64: Add a description on 48-bit address space
 with 4KB pages

On Tuesday, April 29, 2014 11:48 PM, Catalin Marinas wrote:
> On Tue, Apr 29, 2014 at 05:59:27AM +0100, Jungseok Lee wrote:
> > --- a/Documentation/arm64/memory.txt
> > +++ b/Documentation/arm64/memory.txt
> > @@ -8,10 +8,11 @@ This document describes the virtual memory layout
> > used by the AArch64  Linux kernel. The architecture allows up to 4
> > levels of translation  tables with a 4KB page size and up to 3 levels with a 64KB page size.
> >
> > -AArch64 Linux uses 3 levels of translation tables with the 4KB page
> > -configuration, allowing 39-bit (512GB) virtual addresses for both
> > user -and kernel. With 64KB pages, only 2 levels of translation tables
> > are -used but the memory layout is the same.
> > +AArch64 Linux uses 3 levels and 4 levels of translation tables with
> > +the 4KB page configuration, allowing 39-bit (512GB) and 48-bit
> > +(256TB) virtual addresses, respectively, for both user and kernel.
> > +With 64KB pages, only 2 levels of translation tables are used but the
> > +memory layout is the same.
> 
> Any reason why we couldn't use 48-bit address space with 64K pages (implying 3 levels)?

No technical reason.
Since 64K+3levels is not implemented in this set, I didn't add it.

Should 64K+3levels be prepared in this patchset?

> > -AArch64 Linux memory layout with 64KB pages:
> > +AArch64 Linux memory layout with 4KB pages + 4 levels:
> > +
> > +Start			End			Size		Use
> > +-----------------------------------------------------------------------
> > +0000000000000000	0000ffffffffffff	 256TB		user
> > +
> > +ffff000000000000	ffff7bfffffeffff	~124TB		vmalloc
> 
> BTW, maybe as a separate patch we should change the "end" to be exclusive. It becomes harder to modify
> (I've been through this a few times already ;)) and even follow the changes.

Does "exclusive" mean that 0000ffffffffffff is changed to 0001000000000000?
Or Does it mean that "End" column is dropped?

If you are okay, I will make it as a separate patch.

Best Regards
Jungseok Lee

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