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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: WpOUgmaHTKmJgSahNRt/0gJfkqhGIhN/VA5yjNN6qbOKWU8WYJm+kVqhGD+QzkapG3vc9uggP+xNZE8l1bZZTqonWvssyIB0OUbakdM4rgubisoV+y75Apl7Vi2wiM5NRF8JF73cgaQ6aE0i7YkrqE6vLXmYSmaDBKvTp+/ZEpz3ZXx09a9ByF/D+Pat18nNyN6twem/nHKCyWWGJw96iVLh4H/2mSZ+hHNf8VJvM1O8JvWKQYt4G5q8axVC/Pu6o+8Kp5HtkfhIE5GGTipz5myX8upj7KKWTlxMMm9SJ8gNt34uUJo9tmc+qomg7gcDABPuAraA2XXBlSOdqPaOMy7Lgq4M1LmlTEEn7XDCyTMPyDjNbqySYY05fXU8Eub1dnffZcu8TGB2UxajHBw9KgGRjoanm0MqblD22QXoaC2skVkVbOLqLMhcq9Qj8ztv X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 Jun 2026 13:45:35.3836 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c2990695-bac4-473f-a013-08dec62d6217 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: AMS1EPF00000040.eurprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM8PR08MB6370 On 09/06/2026 14:16, Kohei Enju wrote: > On 06/03 12:48, Will Deacon wrote: >> On Tue, Jun 02, 2026 at 04:48:43PM +0100, Suzuki K Poulose wrote: >>> On 02/06/2026 12:01, Will Deacon wrote: >>>> On Tue, May 19, 2026 at 07:12:08PM +0900, Kohei Enju wrote: >>>>> With CONFIG_DEBUG_PREEMPT=y, smp_processor_id() becomes an alias of >>>>> debug_smp_processor_id(). This debug function complains when certain >>>>> conditions that ensure CPU ID stability are not met, specifically when >>>>> it's called from a preemptible context. >>>>> >>>>> In arm_cca_report_new(), which runs in a preemptible context, >>>>> smp_processor_id() triggers a splat [0] due to this. >>>>> >>>>> However, the CPU ID obtained here is used as the target CPU for >>>>> smp_call_function_single() to designate a specific CPU for subsequent >>>>> operations, not to assert that the current thread will continue to >>>>> execute on the same CPU. Therefore, snapshotting the CPU ID itself is >>>>> correct, and thus there's no actual harm except for the splat. >>>>> >>>>> Use raw_smp_processor_id() instead, to directly retrieve the current CPU >>>>> ID without the debug checks, avoiding the unnecessary warning message >>>>> while preserving the correct functional behavior. >>>> >>>> That's pretty disgusting imo so I'd like to see some more justification >>>> for this approach. >>>> >>>>> Note that while migrate_disable() would pin the task to the current CPU, >>>>> this path should not block CPU hotplug events. Therefore, we snapshot >>>>> the current CPU ID and accept that smp_call_function_single() may fail >>>>> if the CPU goes offline. >>>> >>>> Why shouldn't it block CPU hotplug events? What happens if the CPU goes >>>> offline and comes back online again during the loop of continue calls? >>> >>> It need not. It can continue the calls. The RMM keeps track of the internal >>> progress in the "REC" object for this "VCPU". Hotplug ON/OFF >>> doesn't change the REC object in CCA Guest. So, a REC can come back and >>> execute it. But the Linux could fail the operation if the CPU isn't >>> available for fetching the report, after we do a RSI_ATTEST_TOKEN_INIT. >> >> I couldn't really shake that out of the RMM spec tbh: >> RSI_ATTESTATION_TOKEN_CONTINUE is allowed to return RSI_ERROR_UNKNOWN >> and I couldn't find anything about hotplug. Like I said, we are dealing with Virtual CPUs (RECs here) and the only condition that the RMM enforces is the _CONTINUE calls are issued on the same VCPU (REC). Hotplug doesn't change the REC (as a REC cannot be modified or added once the Realm is ACTIVE). >> >> But my main point, really, is why are we not using migrate_disable() >> here? I can't see the justification. There is no reason, why we couldn't use this. TBH, I wasn't aware of the helper ;-) at the time this was initially designed and we didn't want to block CPU hotplug. We could always request a new report, it is not like aborting a report generation has any side effects. > > Hi Will, > Sorry for the late reply. > > I agree that using migrate_disable() makes this path simpler and > clearer. > > I've reviewed the discussion where the original commit was introduced: > https://lore.kernel.org/linux-arm-kernel/7a83461d-40fd-4e61-8833-5dae2abaf82b@arm.com/ > but I couldn't find a strong reason why we shouldn't block CPU hotplug > or use migrate_disable(), even though I can see the current design was > intentional. > > So I'm happy to rework the patch to use migrate_disable() and remove > the smp_call_function_single() calls if there are no objections. I am fine with removing with migrate_disable() approach. Cheers Suzuki > >> >> Will >>