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

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Date:	Mon, 29 Feb 2016 11:35:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	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>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread

On Sun, Feb 28, 2016 at 02:32:28PM +0000, Mathieu Desnoyers wrote:
> The part of ABI I'm trying to express here is for discoverability
> of available features by user-space. For instance, a kernel
> could be configured with "CONFIG_RSEQ=n", and userspace should
> not rely on the rseq fields of the thread-local ABI in that case.

Per the just proposed interface; discoverability would end with:

	thread_local_abi_register(NULL, TLA_ENABLE_RSEQ, 0);

failing. This would indicate your kernel does not support (or your glibc
failed to register, depending on error code I suppose).

Then your program can either fall back to full atomics or just bail.