linux-kernel - Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

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Message-Id: <201512212313.34114.arnd@arndb.de>
Date:	Mon, 21 Dec 2015 23:13:33 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Dr. Philipp Tomsich" <philipp.tomsich@...obroma-systems.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Andrew Pinski <pinskia@...il.com>,
	"Joseph S. Myers" <joseph@...esourcery.com>,
	"Kapoor, Prasun" <Prasun.Kapoor@...iumnetworks.com>,
	broonie@...nel.org, Nathan Lynch <Nathan_Lynch@...tor.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Graf <agraf@...e.de>,
	Alexey Klimov <klimov.linux@...il.com>,
	Yury Norov <ynorov@...iumnetworks.com>,
	Jan Dakinevich <jan.dakinevich@...il.com>,
	Andrew Pinski <apinski@...ium.com>,
	David Daney <ddaney.cavm@...il.com>,
	Andreas Schwab <schwab@...e.de>,
	"Zhangjian (Bamvor)" <bamvor.zhangjian@...wei.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Christoph Müllner 
	<christoph.muellner@...obroma-systems.com>,
	Marcus Shawcroft <Marcus.Shawcroft@....com>
Subject: Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

On Monday 21 December 2015, Dr. Philipp Tomsich wrote:
> 
> > On 18 Dec 2015, at 13:47, Arnd Bergmann <arnd@...db.de> wrote:
> > 
> >> 3. Follow the PCS up to glibc but always pass syscall arguments in W
> >>   registers, like AArch32 compat support (the least preferred option,
> >>   the only advantage is a single wrapper for all syscalls but it would
> >>   be doing unnecessary zeroing even for syscalls where it isn't needed)
> > 
> > This would mean we cannot pass 64-bit arguments in registers, right?
> 
> Note that there’s no 32bit registers (the ‘w’-form always refers to the lower
> 32bits of a 64bit register, with implicit zero-extension)… and load/store
> instructions always use the full base-register (‘x’-form) for address calculation.
> I.e. a load/store would inadvertently pickup “random garbage” in the upper 
> 32bits, if no explicit zero-extension is applied.
> 
> In other words: all zero-extensions for 32bit arguments should be explicit
> on the kernel side.

I think that is what Catalin meant with the single wrapper in the description:
the kernel always zeroes out the upper 32 bits in the initial trap, and then
we only need to do sign-extensions for the few cases that need it, and pass 64-bit
arguments in two lower halves. In contrast, approach 1 adds a separate wrapper
for each syscall that takes care of both sign-extending where necessary and
zero-extending all othe 32-bit arguments but not the 64-bit arguments.

	Arnd
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