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.

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.

[0]
 BUG: using smp_processor_id() in preemptible [00000000] code: cca-workload-at/134
 caller is debug_smp_processor_id+0x20/0x2c
 CPU: 0 UID: 0 PID: 134 Comm: cca-workload-at Not tainted 7.0.0-rc1-gc74a64d12073 #1 PREEMPT
 Hardware name: linux,dummy-virt (DT)
 Call trace:
  [...]
  check_preemption_disabled+0xf8/0x100
  debug_smp_processor_id+0x20/0x2c
  arm_cca_report_new+0x54/0x230
  tsm_report_read+0x184/0x260
  tsm_report_outblob_read+0x18/0x38
  configfs_bin_read_iter+0xf4/0x1dc
  vfs_read+0x230/0x31c
  [...]

Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 7999edc484ca ("virt: arm-cca-guest: TSM_REPORT support for realms")
Signed-off-by: Kohei Enju <enju.kohei@fujitsu.com>
---
 drivers/virt/coco/arm-cca-guest/arm-cca-guest.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c b/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
index 0c9ea24a200c..b463bf35bf30 100644
--- a/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
+++ b/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
@@ -107,8 +107,15 @@ static int arm_cca_report_new(struct tsm_report *report, void *data)
 	 * instead of simply calling get_cpu() because of the need to
 	 * allocate outblob based on the returned value from the 'init'
 	 * call and that cannot be done in an atomic context.
+	 *
+	 * 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.
+	 * Any resulting error is propagated to user-space, which is
+	 * expected to handle it.
 	 */
-	cpu = smp_processor_id();
+	cpu = raw_smp_processor_id();
 
 	info.challenge = desc->inblob;
 	info.challenge_size = desc->inblob_len;
-- 
2.47.3