From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753311AbcCAVcQ (ORCPT <rfc822;w@1wt.eu>);
	Tue, 1 Mar 2016 16:32:16 -0500
Received: from casper.infradead.org ([85.118.1.10]:45063 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751133AbcCAVcP (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 1 Mar 2016 16:32:15 -0500
Date: Tue, 1 Mar 2016 22:32:02 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ben Maurer <bmaurer@fb.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Russell King <linux@arm.linux.org.uk>,
        linux-api <linux-api@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Dave Watson <davejwatson@fb.com>, rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Will Deacon <will.deacon@arm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Chris Lameter <cl@linux.com>, Andi Kleen <andi@firstfloor.org>,
        Josh Triplett <josh@joshtriplett.org>, Paul Turner <pjt@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Hunter <ahh@google.com>
Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
 running thread
Message-ID: <20160301213202.GY6357@twins.programming.kicks-ass.net>
References: <1456270120-7560-1-git-send-email-mathieu.desnoyers@efficios.com>
 <CA+55aFysWipEagSdgeXbLx+Au4vomvyttdNFJtM+HffmBECQiA@mail.gmail.com>
 <1401667361.10273.1456617236327.JavaMail.zimbra@efficios.com>
 <CA+55aFwcgwRxvVBz5kk_3O8dESXAGJ4KHBkf=pSXjiS7Xh4NwA@mail.gmail.com>
 <1082926946.10326.1456619994590.JavaMail.zimbra@efficios.com>
 <CA+55aFwZY1BR5NdjAP-rytd29hE2EZEXgcaBKjepWFFj+oR1SA@mail.gmail.com>
 <1538518747.10504.1456669948568.JavaMail.zimbra@efficios.com>
 <20160229103506.GJ6356@twins.programming.kicks-ass.net>
 <676569856.13488.1456863792603.JavaMail.zimbra@efficios.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <676569856.13488.1456863792603.JavaMail.zimbra@efficios.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.

> struct thread_local_abi {
>         /*
>          * Thread-local ABI cpu_id field.
>          * Updated by the kernel, and read by user-space with
>          * single-copy atomicity semantics. Aligned on 32-bit.
>          * Values:
>          * >= 0: CPU number of running thread.
>          * -1 (initial value): means the cpu_id feature is inactive.
>          * -2: cpu_id feature is not available.
>          */
>         int32_t cpu_id;
> 
>         /*
>          * Thread-local ABI rseq_seqnum field.
>          * Updated by the kernel, and read by user-space with
>          * single-copy atomicity semantics. Aligned on 32-bit.
>          * Values:
>          * >= 0: current seqnum for this thread (feature is active).
>          * -1 (initial value): means the rseq feature is inactive.
>          * -2: rseq feature is not available.
>          */
>         int32_t rseq_seqnum;

So I really hate that, that makes we have to check for these special
values whenever we increment the seq count and cannot have it wrap
naturally.