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Mon, 25 Nov 2024 13:40:23 +0000 (GMT) Date: Mon, 25 Nov 2024 14:40:22 +0100 From: Heiko Carstens To: Claudio Imbrenda Cc: Christian Borntraeger , Janosch Frank , David Hildenbrand , kvm@vger.kernel.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] KVM: s390: Increase size of union sca_utility to four bytes Message-ID: <20241125134022.14417-E-hca@linux.ibm.com> References: <20241125115039.1809353-1-hca@linux.ibm.com> <20241125115039.1809353-4-hca@linux.ibm.com> <20241125132042.44918953@p-imbrenda> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241125132042.44918953@p-imbrenda> X-TM-AS-GCONF: 00 X-Proofpoint-GUID: ZAMo7C4kmPJKVx8m5_WxSnGl_7IhNwW1 X-Proofpoint-ORIG-GUID: ZAMo7C4kmPJKVx8m5_WxSnGl_7IhNwW1 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1051,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-10-15_01,2024-10-11_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 clxscore=1015 impostorscore=0 priorityscore=1501 mlxlogscore=549 malwarescore=0 adultscore=0 phishscore=0 lowpriorityscore=0 mlxscore=0 bulkscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2409260000 definitions=main-2411250115 On Mon, Nov 25, 2024 at 01:20:42PM +0100, Claudio Imbrenda wrote: > On Mon, 25 Nov 2024 12:50:39 +0100 > Heiko Carstens wrote: > > > kvm_s390_update_topology_change_report() modifies a single bit within > > sca_utility using cmpxchg(). Given that the size of the sca_utility union > > is two bytes this generates very inefficient code. Change the size to four > > bytes, so better code can be generated. > > > > Even though the size of sca_utility doesn't reflect architecture anymore > > this seems to be the easiest and most pragmatic approach to avoid > > inefficient code. > > wouldn't an atomic bit_op be better in that case? I had that, but decided against it, since the generated code isn't shorter. And it would require and unsigned long type within the union, or a cast, which I also both disliked.