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

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Message-ID: <20140429144759.GL17007@arm.com>
Date:	Tue, 29 Apr 2014 15:47:59 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Jungseok Lee <jays.lee@...sung.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	"steve.capper@...aro.org" <steve.capper@...aro.org>,
	"sungjinn.chung@...sung.com" <sungjinn.chung@...sung.com>,
	Arnd Bergmann <arnd@...db.de>,
	"kgene.kim@...sung.com" <kgene.kim@...sung.com>,
	"ilho215.lee@...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 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)?

> -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.

-- 
Catalin
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