From owner-svn-src-head@freebsd.org  Thu Sep 22 09:28:17 2016
Return-Path: <owner-svn-src-head@freebsd.org>
Delivered-To: svn-src-head@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id BC692BE35A0;
 Thu, 22 Sep 2016 09:28:17 +0000 (UTC)
 (envelope-from kostikbel@gmail.com)
Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1])
 (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits))
 (Client did not present a certificate)
 by mx1.freebsd.org (Postfix) with ESMTPS id 3C871DCE;
 Thu, 22 Sep 2016 09:28:17 +0000 (UTC)
 (envelope-from kostikbel@gmail.com)
Received: from tom.home (kib@localhost [127.0.0.1])
 by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id u8M9S8TO063387
 (version=TLSv1 cipher=DHE-RSA-CAMELLIA256-SHA bits=256 verify=NO);
 Thu, 22 Sep 2016 12:28:08 +0300 (EEST)
 (envelope-from kostikbel@gmail.com)
DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua u8M9S8TO063387
Received: (from kostik@localhost)
 by tom.home (8.15.2/8.15.2/Submit) id u8M9S85q063386;
 Thu, 22 Sep 2016 12:28:08 +0300 (EEST)
 (envelope-from kostikbel@gmail.com)
X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com
 using -f
Date: Thu, 22 Sep 2016 12:28:08 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Ed Schouten <ed@FreeBSD.org>
Cc: src-committers@freebsd.org, svn-src-all@freebsd.org,
 svn-src-head@freebsd.org
Subject: Re: svn commit: r306162 - in head/sys/arm: arm include
Message-ID: <20160922092808.GR38409@kib.kiev.ua>
References: <201609220815.u8M8F0Lo096297@repo.freebsd.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <201609220815.u8M8F0Lo096297@repo.freebsd.org>
User-Agent: Mutt/1.6.1 (2016-04-27)
X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00,
 DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no
 autolearn_force=no version=3.4.1
X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home
X-BeenThere: svn-src-head@freebsd.org
X-Mailman-Version: 2.1.23
Precedence: list
List-Id: SVN commit messages for the src tree for head/-current
 <svn-src-head.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/svn-src-head>,
 <mailto:svn-src-head-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/svn-src-head/>
List-Post: <mailto:svn-src-head@freebsd.org>
List-Help: <mailto:svn-src-head-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/svn-src-head>,
 <mailto:svn-src-head-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 22 Sep 2016 09:28:17 -0000

On Thu, Sep 22, 2016 at 08:15:00AM +0000, Ed Schouten wrote:
> Author: ed
> Date: Thu Sep 22 08:14:59 2016
> New Revision: 306162
> URL: https://svnweb.freebsd.org/changeset/base/306162
> 
> Log:
>   Make it possible to safely use TPIDRURW from userspace.
>   
>   On amd64, arm64 and i386, we have the possibility to switch between TLS
>   areas in userspace. The nice thing about this is that it makes it easier
>   to do light-weight threading, if we ever feel like doing that. On armv6,
>   let's go into the same direction by making it possible to safely use the
>   TPIDRURW register, which is intended for this purpose.

What do you mean about 'possibility to switch between TLS areas in
usermode' ? On x86, kernel switches %fs/%gs bases on the context
switches. There is, indeed, a way to turn off this functionality, but it
is somewhat unobvious, I have to admit.

E.g. on Ivy+, RDFSBASE/RDGSBASE work.  WRFSBASE/WRGSBASE are enabled,
but they are de-facto not functional, since next context switch overrides
the base, in default setup.