From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 311E6C433E0 for ; Wed, 27 May 2020 13:03:25 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 00D0B20873 for ; Wed, 27 May 2020 13:03:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="XYrGYTz2" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 00D0B20873 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jdvi9-0002rG-OH; Wed, 27 May 2020 13:03:09 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jdvi8-0002rB-Hl for xen-devel@lists.xenproject.org; Wed, 27 May 2020 13:03:08 +0000 X-Inumbo-ID: 68fd1f60-a01a-11ea-9947-bc764e2007e4 Received: from us-smtp-1.mimecast.com (unknown [205.139.110.61]) by us1-rack-iad1.inumbo.com (Halon) with ESMTP id 68fd1f60-a01a-11ea-9947-bc764e2007e4; Wed, 27 May 2020 13:03:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590584587; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type; bh=oy0V2qQifVRxyl3hnuRXtetKes4TGaiAhxy8832yoas=; b=XYrGYTz2e4jmKUSiAK9jElMI1dgb4uDJ5Bdic9uHARTUdIYiEeZGZPHajDPg8SckayNqNu spZwhiNa6HAQeTMkYys74SlloaZ1ACCZMsSQOfdEQbWBLV+DkzHrOo0eCSQ3LXDJR0KdKx ZNgkSymDzwoMeCVdBo3VETu6fuVGn1Y= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-218-tGzjnUSgMNe2-8nuD3ru8Q-1; Wed, 27 May 2020 09:03:04 -0400 X-MC-Unique: tGzjnUSgMNe2-8nuD3ru8Q-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DEE0D460; Wed, 27 May 2020 13:03:03 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-113-106.ams2.redhat.com [10.36.113.106]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3622760C05; Wed, 27 May 2020 13:03:03 +0000 (UTC) From: Florian Weimer To: xen-devel@lists.xenproject.org Subject: -mno-tls-direct-seg-refs support in glibc for i386 PV Xen Date: Wed, 27 May 2020 15:03:01 +0200 Message-ID: <87mu5til8a.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: libc-alpha@sourceware.org Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" I'm about to remove nosegneg support from upstream glibc, special builds that use -mno-tls-direct-seg-refs, and the ability load different libraries built in this mode automatically, when the Linux kernel tells us to do that. I think the intended effect is that these special builds do not use operands of the form %gs:(%eax) when %eax has the MSB set because that had a performance hit with paravirtualization on 32-bit x86. Instead, the thread pointer is first loaded from %gs:0, and the actual access does not use a segment prefix. Before doing that, I'd like to ask if anybody is still using this feature? I know that we've been carrying nosegneg libraries for many years, in some cases even after we stopped shipping 32-bit kernels. 8-/ The feature has always been rather poorly documented, and the way the dynamic loader selects those nosegneg library variants is still very bizarre. Thanks, Florian