From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com [148.163.158.5]) by sourceware.org (Postfix) with ESMTPS id B72E7383F953 for ; Mon, 23 May 2022 14:19:51 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org B72E7383F953 Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.17.1.5/8.17.1.5) with ESMTP id 24NEDjnV002727; Mon, 23 May 2022 14:19:51 GMT Received: from pps.reinject (localhost [127.0.0.1]) by mx0b-001b2d01.pphosted.com (PPS) with ESMTPS id 3g8buc049q-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 23 May 2022 14:19:50 +0000 Received: from m0098419.ppops.net (m0098419.ppops.net [127.0.0.1]) by pps.reinject (8.17.1.5/8.17.1.5) with ESMTP id 24NEIRgw022590; Mon, 23 May 2022 14:19:50 GMT Received: from ppma02dal.us.ibm.com (a.bd.3ea9.ip4.static.sl-reverse.com [169.62.189.10]) by mx0b-001b2d01.pphosted.com (PPS) with ESMTPS id 3g8buc049j-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 23 May 2022 14:19:50 +0000 Received: from pps.filterd (ppma02dal.us.ibm.com [127.0.0.1]) by ppma02dal.us.ibm.com (8.16.1.2/8.16.1.2) with SMTP id 24NEIpIN022605; Mon, 23 May 2022 14:19:49 GMT Received: from b03cxnp07027.gho.boulder.ibm.com (b03cxnp07027.gho.boulder.ibm.com [9.17.130.14]) by ppma02dal.us.ibm.com with ESMTP id 3g6qq9crek-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 23 May 2022 14:19:49 +0000 Received: from b03ledav005.gho.boulder.ibm.com (b03ledav005.gho.boulder.ibm.com [9.17.130.236]) by b03cxnp07027.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 24NEJmN428901768 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 23 May 2022 14:19:48 GMT Received: from b03ledav005.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8BCECBE051; Mon, 23 May 2022 14:19:48 +0000 (GMT) Received: from b03ledav005.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 05005BE056; Mon, 23 May 2022 14:19:48 +0000 (GMT) Received: from [9.160.107.32] (unknown [9.160.107.32]) by b03ledav005.gho.boulder.ibm.com (Postfix) with ESMTP; Mon, 23 May 2022 14:19:47 +0000 (GMT) Message-ID: <27e05114-fd3a-b7cc-1bdd-05d8eaf0b483@linux.ibm.com> Date: Mon, 23 May 2022 09:19:47 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [RFC Linux patch] powerpc: add documentation for HWCAPs Content-Language: en-US To: Nicholas Piggin , linuxppc-dev@lists.ozlabs.org Cc: gcc@gcc.gnu.org, libc-alpha@sourceware.org References: <20220520051528.98097-1-npiggin@gmail.com> <1653091346.1a5h1ae3pd.astroid@bobo.none> From: Paul E Murphy In-Reply-To: <1653091346.1a5h1ae3pd.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: iwiQ7e3rQkxS3vx6qxmaMpdUasbj9GXf X-Proofpoint-GUID: v1u-EZaBbZFjIMDskUdp7D2rBgkdccIa X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.874,Hydra:6.0.486,FMLib:17.11.64.514 definitions=2022-05-23_06,2022-05-23_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 malwarescore=0 clxscore=1015 priorityscore=1501 suspectscore=0 adultscore=0 impostorscore=0 mlxscore=0 bulkscore=0 phishscore=0 lowpriorityscore=0 mlxlogscore=975 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2202240000 definitions=main-2205230079 X-Spam-Status: No, score=-6.6 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_EF, NICE_REPLY_A, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gcc@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 23 May 2022 14:19:53 -0000 On 5/20/22 7:11 PM, Nicholas Piggin wrote: > Excerpts from Paul E Murphy's message of May 21, 2022 12:21 am: >> >> >> On 5/20/22 12:15 AM, Nicholas Piggin via Gcc wrote: >>> +PPC_FEATURE2_TAR >>> + VSX facility is available. >> >> Was manipulating the tar spr was once a privileged instruction, is this >> a hint userspace can use the related instructions? > > It can be disabled with facility control, and I guess there was > some consideration for how it might be used, e.g., "system software" > could use it for its own purpose then clear the bit for the application. > > In practice I don't really know what makes use of this or whether > anything sanely can, it's marked reserved in the ABI. Would be > interesting to know whether there is much benefit to use it in the > compiler. The kernel could actually use it for something nifty if we > were able to prevent userspace from accessing it entirely... It might be useful as a scratch register for indirect branches in some odd cases, such as golang's preemptive userspace threading. Though, it seems more trouble than its worth for a very limited benefit. > >>> + >>> +PPC_FEATURE2_HAS_IEEE128 >>> + IEEE 128 is available? What instructions/data? >> >> Maybe something like "IEEE 128 binary floating point instructions are >> supported. Individual instruction availability is dependent on the >> reported architecture version."? > > Right, I just didn't know what architectural class of instructions > those are. Is it just VSX in general or are there some specific > things we can name? I think ISA 3.1 buckets this into an OpenPOWER Linux Optional Feature for "Quad-precision floating-point (QFP)". I guess ISA 3.0 predates those categorizations. >>> +PPC_FEATURE2_MMA >>> + MMA facility is available. >> >> Maybe another note that specific instruction availability may depend on >> the reported architecture version? Yep. I wonder if it would help to note how these align (or don't) with the various OpenPOWER features.