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Tsirkin" , Cornelia Huck References: <20240910175809.2135596-1-david@redhat.com> <20240910175809.2135596-8-david@redhat.com> <6636c963-228f-4bea-87c5-bd4f75521c75@redhat.com> <20240927200525.5a90f172.pasic@linux.ibm.com> <10165d22-c3e8-4db1-9874-8b63ca59afe9@linux.ibm.com> <20240930145712.526a1c79.pasic@linux.ibm.com> <972044f1-62e4-4ac0-8b24-e0bb78770309@linux.ibm.com> <20241001153132.08b0dca9.pasic@linux.ibm.com> Content-Language: en-US From: Christian Borntraeger In-Reply-To: <20241001153132.08b0dca9.pasic@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: EIXRmopc9ZPMyRg_fScy4q7eHVtBRLR_ X-Proofpoint-GUID: y7fzWI23JOzBkEUWfEY_QZEMjy5bjMit 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=2024-10-01_11,2024-09-30_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 adultscore=0 suspectscore=0 spamscore=0 lowpriorityscore=0 impostorscore=0 mlxlogscore=753 clxscore=1015 mlxscore=0 priorityscore=1501 phishscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2408220000 definitions=main-2410010092 Received-SPF: pass client-ip=148.163.158.5; envelope-from=borntraeger@linux.ibm.com; helo=mx0b-001b2d01.pphosted.com X-Spam_score_int: -26 X-Spam_score: -2.7 X-Spam_bar: -- X-Spam_report: (-2.7 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 01.10.24 um 15:31 schrieb Halil Pasic: > On Tue, 1 Oct 2024 11:15:02 +0200 > Christian Borntraeger wrote: > [..] >>>> So 500+4 should probably not cause any harm apart from branch prediction >>>> going wrong the first 2 or 3 notifies. >>>> >>>> 502 will make kvm_s390_handle_diag larger. >>> >>> What do you mean by this last paragraph? > [..] > >> gcc has logic for switch statements that decide about branch table or >> a chained compare+jump. I think due to spectre gcc now avoids indirect >> branches as much as possible but still a larger switch statement might >> kick the decision from inline compare/jump to a branch table. >> >> I am not worried in this particular case this was more or less a >> "what could go wrong". > > Hm, you did state that "502 will make kvm_s390_handle_diag larger". I > suppose now we agree that 502 would not make kvm_s390_handle_diag larger. > Right? > > I understood that you prefer 500+4 over 502 because the latter would > make kvm_s390_handle_diag larger. Now that we have, I hope clarified, > that 502 would not make the switch larger, do you still prefer 500+4? > > BTW your insights are very appreciated! OK you mean that diag502 is not handled in the kernel but instead via default. Yes you are right. So it should not matter I guess.