linux-kernel - Re: [PATCH v6 14/19] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

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Message-ID: <20151203174708.GO10747@e104818-lin.cambridge.arm.com>
Date:	Thu, 3 Dec 2015 17:47:08 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Yury Norov <ynorov@...iumnetworks.com>
Cc:	Arnd Bergmann <arnd@...db.de>, pinskia@...il.com,
	Prasun.Kapoor@...iumnetworks.com, Nathan_Lynch@...tor.com,
	linux-kernel@...r.kernel.org, agraf@...e.de,
	klimov.linux@...il.com, broonie@...nel.org,
	jan.dakinevich@...il.com, joseph@...esourcery.com,
	ddaney.cavm@...il.com, schwab@...e.de, bamvor.zhangjian@...wei.com,
	philipp.tomsich@...obroma-systems.com, andrey.konovalov@...aro.org,
	linux-arm-kernel@...ts.infradead.org,
	christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH v6 14/19] arm64:ilp32: add sys_ilp32.c and a separate
 table (in entry.S) to use it

On Wed, Dec 02, 2015 at 12:29:04AM +0300, Yury Norov wrote:
> My question. Why aarch64 defines COMPAT_SHMLBA as 0x4000?

This was done to match the arch/arm value of 4 * 4K. The historical
32-bit reason for 4 pages is to cope with aliasing VIPT caches (see
https://git.kernel.org/cgit/linux/kernel/git/tglx/history.git/commit?id=4197692eef113eeb8e3e413cc70993a5e667e5b8)

> If there's
> no specific reason for it, it looks like a bug, and we should
> define it like in arch/arm:
>         #define SHMLBA  (4 * PAGE_SIZE)          /* attach addr a multiple of this */

I guess you meant COMPAT_SHMLBA. I'm not sure there is much value in
keeping 4*PAGE_SIZE for larger page sizes but I agree that the current
16K value doesn't work well with 64K pages.

-- 
Catalin
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