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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Nov 2025 14:19:50.4153 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ebaf0a16-aedb-4bbf-58e3-08de1e08b697 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB1PEPF000509FD.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DBAPR08MB5656 Hi Arnd. > On Thu, Nov 6, 2025, at 10:40, Yeoreum Yun wrote: > > apply FEAT_LSUI instruction to emulate deprecated swpX instruction. > > Can you explain in the changrelog why you do this? > > In particular, is this a performance optimization or is this required > for correctness in some scenario? I think the main purpose using for this is to "remove the toggling the PSTATE.PAN" bit. so that remove some *potential* problem which can happen when PAN bit is cleared. also, though swpb might add some complexity, but swp can get a little bit of benifit by removing the ll/sc way and reduce the amount of instructions. > I would have expected that there is very little benefit in > changing the swp/swpb emulation here if the existing code > has to remain compiled into the kernel and the overhead of > the trap is so much larger than the uaccess_enable_privileged() > overhead. Yeap. since the *trap* overhead is much larger than the uaccess_enable_privileged() in the performnace perspecitve, It would has a little effect. However, as I mention above, the main purpose of this is to remove toggling the PSTATE.PAN if possible. there would be a little bit of additional commplexity for swpb but it seems affordable. Thanks. [...] -- Sincerely, Yeoreum Yun