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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Date:	Thu, 3 Dec 2015 21:14:41 +0300
From:	Yury Norov <ynorov@...iumnetworks.com>
To:	Catalin Marinas <catalin.marinas@....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 Thu, Dec 03, 2015 at 05:47:08PM +0000, Catalin Marinas wrote:
> 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 citated arm code here. In aarch64 it's COMPAT_SHMLBA, of course.

> 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.

Arnd told there will be a workaround for arm v6 caches. Than this
header will not be needed at all. Until that, this is simpliest
fix as it doesn't affect userspace.

> 
> -- 
> Catalin
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