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Date: Wed, 5 Dec 2018 17:36:52 +0000
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH V4 2/6] arm64: mm: Introduce DEFAULT_MAP_WINDOW
Message-ID: <20181205173651.GD27881@arrakis.emea.arm.com>
References: <20181205164145.24568-1-steve.capper@arm.com>
 <20181205164145.24568-3-steve.capper@arm.com>
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To: Steve Capper <steve.capper@arm.com>
Cc: linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, will.deacon@arm.com, jcm@redhat.com, ard.biesheuvel@linaro.org

On Wed, Dec 05, 2018 at 04:41:41PM +0000, Steve Capper wrote:
> We wish to introduce a 52-bit virtual address space for userspace but
> maintain compatibility with software that assumes the maximum VA space
> size is 48 bit.
> 
> In order to achieve this, on 52-bit VA systems, we make mmap behave as
> if it were running on a 48-bit VA system (unless userspace explicitly
> requests a VA where addr[51:48] != 0).
> 
> On a system running a 52-bit userspace we need TASK_SIZE to represent
> the 52-bit limit as it is used in various places to distinguish between
> kernelspace and userspace addresses.
> 
> Thus we need a new limit for mmap, stack, ELF loader and EFI (which uses
> TTBR0) to represent the non-extended VA space.
> 
> This patch introduces DEFAULT_MAP_WINDOW and DEFAULT_MAP_WINDOW_64 and
> switches the appropriate logic to use that instead of TASK_SIZE.
> 
> Signed-off-by: Steve Capper <steve.capper@arm.com>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>