linux-kernel - Re: [PATCH 01/23] all: syscall wrappers: add documentation

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Date:	Wed, 25 May 2016 14:28:21 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	arnd@...db.de
Cc:	ynorov@...iumnetworks.com, catalin.marinas@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, libc-alpha@...rceware.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	pinskia@...il.com, broonie@...nel.org, joseph@...esourcery.com,
	christoph.muellner@...obroma-systems.com,
	bamvor.zhangjian@...wei.com, szabolcs.nagy@....com,
	klimov.linux@...il.com, Nathan_Lynch@...tor.com, agraf@...e.de,
	Prasun.Kapoor@...iumnetworks.com, kilobyte@...band.pl,
	geert@...ux-m68k.org, philipp.tomsich@...obroma-systems.com
Subject: Re: [PATCH 01/23] all: syscall wrappers: add documentation

From: Arnd Bergmann <arnd@...db.de>
Date: Wed, 25 May 2016 23:01:06 +0200

> On Wednesday, May 25, 2016 1:50:39 PM CEST David Miller wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>> Date: Wed, 25 May 2016 22:47:33 +0200
>> 
>> > If we use the normal calling conventions, we could remove these overrides
>> > along with the respective special-case handling in glibc. None of them
>> > look particularly performance-sensitive, but I could be wrong there.
>> 
>> You could set the lowest bit in the system call entry pointer to indicate
>> the upper-half clears should be elided.
> 
> Right, but that would introduce an extra conditional branch in the syscall
> hotpath, and likely eliminate the gains from passing the loff_t arguments
> in a single register instead of a pair.

Ok, then, how much are you really gaining from avoiding a 'shift' and
an 'or' to build the full 64-bit value?  3 cycles?  Maybe 4?

And the executing the wrappers, those have a non-trivial cost too.

Cost wise, this seems like it all cancels out in the end, but what
do I know?