linux-kernel - Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

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Message-ID: <56D60DFA.4040809@zytor.com>
Date:	Tue, 1 Mar 2016 13:47:38 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ben Maurer <bmaurer@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	linux-api <linux-api@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Dave Watson <davejwatson@...com>,
	rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Chris Lameter <cl@...ux.com>, Andi Kleen <andi@...stfloor.org>,
	Josh Triplett <josh@...htriplett.org>,
	Paul Turner <pjt@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Andrew Hunter <ahh@...gle.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread

On 03/01/16 13:32, Peter Zijlstra wrote:
> On Tue, Mar 01, 2016 at 08:23:12PM +0000, Mathieu Desnoyers wrote:
>> I think it's important that user-space fast-paths can quickly
>> detect whether the feature is enabled without having to rely on
>> always reading a separate cache-line. I've put together an ABI
>> proposal that take into account the feedback received so far.
> 
> Nah, adding detectoring code to fast paths is silly, makes them less
> fast. Doesn't userspace have self modifying code? I know that at least
> glibc does linker trickery to call different functions depending on
> runtime context.
> 

No, userspace does not have self-modifying code.  The glibc indirect
function is done at dynamic link time; it is also worth noting that
resolving global symbols through dynamic linking often requires an
indirect call.

	-hpa