From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=nAH3=MZ=lists.ozlabs.org=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@kernel.org>
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=-5.2 required=3.0 tests=BAYES_00,
	HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,
	USER_AGENT_SANE_1 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 BB326C4338F
	for <linuxppc-dev@archiver.kernel.org>; Mon,  2 Aug 2021 18:10:44 +0000 (UTC)
Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117])
	(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 0B0C360EE3
	for <linuxppc-dev@archiver.kernel.org>; Mon,  2 Aug 2021 18:10:44 +0000 (UTC)
DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 0B0C360EE3
Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org
Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=lists.ozlabs.org
Received: from boromir.ozlabs.org (localhost [IPv6:::1])
	by lists.ozlabs.org (Postfix) with ESMTP id 4GdmJf3b65z3cRh
	for <linuxppc-dev@archiver.kernel.org>; Tue,  3 Aug 2021 04:10:42 +1000 (AEST)
Authentication-Results: lists.ozlabs.org;
 spf=permerror (SPF Permanent Error: Unknown mechanism
 found: ip:192.40.192.88/32) smtp.mailfrom=kernel.crashing.org
 (client-ip=63.228.1.57; helo=gate.crashing.org;
 envelope-from=segher@kernel.crashing.org; receiver=<UNKNOWN>)
Received: from gate.crashing.org (gate.crashing.org [63.228.1.57])
 by lists.ozlabs.org (Postfix) with ESMTP id 4GdmJH4NkBz307B
 for <linuxppc-dev@lists.ozlabs.org>; Tue,  3 Aug 2021 04:10:23 +1000 (AEST)
Received: from gate.crashing.org (localhost.localdomain [127.0.0.1])
 by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 172I77YW031222;
 Mon, 2 Aug 2021 13:07:07 -0500
Received: (from segher@localhost)
 by gate.crashing.org (8.14.1/8.14.1/Submit) id 172I74h0031211;
 Mon, 2 Aug 2021 13:07:04 -0500
X-Authentication-Warning: gate.crashing.org: segher set sender to
 segher@kernel.crashing.org using -f
Date: Mon, 2 Aug 2021 13:07:04 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to
 generic C implementation.)
Message-ID: <20210802180704.GO1583@gate.crashing.org>
References: <b5f948b4-759d-bb9f-06aa-6c15d37cd2bb@molgen.mpg.de>
 <a273c619-9258-e29a-24c3-ea47a13c4817@molgen.mpg.de>
 <3661999754da1a5e5c810fa669654cc7db95b059.camel@kernel.crashing.org>
 <4f037af0-5066-ebb9-53a6-733b3bd8eeac@molgen.mpg.de>
 <878s1q1udj.fsf@mpe.ellerman.id.au>
 <OF44F7146F.67A4C1C2-ON00258720.004DBF64-00258720.004FCFCC@ibm.com>
 <87czqwl67h.fsf@mpe.ellerman.id.au>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87czqwl67h.fsf@mpe.ellerman.id.au>
User-Agent: Mutt/1.4.2.3i
X-BeenThere: linuxppc-dev@lists.ozlabs.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Linux on PowerPC Developers Mail List <linuxppc-dev.lists.ozlabs.org>
List-Unsubscribe: <https://lists.ozlabs.org/options/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=unsubscribe>
List-Archive: <http://lists.ozlabs.org/pipermail/linuxppc-dev/>
List-Post: <mailto:linuxppc-dev@lists.ozlabs.org>
List-Help: <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=help>
List-Subscribe: <https://lists.ozlabs.org/listinfo/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=subscribe>
Cc: parkerderek86@gmail.com, pmenzel@molgen.mpg.de, laboger@linux.vnet.ibm.com,
 xaionaro@gmail.com, Paul Murphy <murp@ibm.com>, paulus@samba.org,
 murphyp@linux.ibm.com, linuxppc-dev@lists.ozlabs.org
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Mon, Aug 02, 2021 at 04:18:58PM +1000, Michael Ellerman wrote:
> > Go up to this point has only used the vdso function __kernel_clock_gettime; it
> > is the only entry point which would need to explicitly avoid R30 for
> > Go's sake.
> 
> I thought about limiting the workaround to just that code, but it seemed
> silly and likely to come back to bite us. Once the compilers decides to
> spill a non-volatile there are plenty of other ones to choose from.

It can be cheaper to spill N..31 consecutively, using stmw for example.
For 64-bit Power implementations it doesn't currently make any
difference.  Since none of this will be inlined it doesn't have any real
impact.

(This also happens with -m32 -fpic, which always sets GPR30 as fixed, it
is the offset table register.  With those flags -ffixed-r30 doesn't do
anything btw (r30 already *is* a fixed function register), and this will
not work with a Go that clobbers r30.  But this is academic :-) )


Segher