[PULL 1/5] ppc/spapr: Skip system reset for quiesced CPUs

[Harsh Prateek Bora <harshpb@linux.ibm.com>]

From: Shivang Upadhyay <shivangu@linux.ibm.com>

During DLPAR CPU hotplug, newly added CPUs start in RTAS stopped state
(quiesced). If a kexec crash occurs before the guest starts these CPUs
via start-cpu RTAS call, H_SIGNAL_SYS_RESET_ALL_OTHERS will reset them
anyway, causing the kdump kernel to hang:

  [    5.519483][    T1] Processor 0 is stuck.
  [   11.089481][    T1] Processor 1 is stuck.

The hypervisor should only reset CPUs that the guest has started. The
cpu->env.quiesced flag tracks RTAS stopped state - CPUs in this state
are already inactive and should not be reset.

Skip system reset for quiesced CPUs to prevent kdump hangs during CPU
hotplug operations.

Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: Harsh Prateek Bora <harshpb@linux.ibm.com>
Cc: Mahesh J Salgaonkar <mahesh@linux.ibm.com>
Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
Suggested-by: Vishal Chourasia <vishalc@linux.ibm.com>
Reviewed-by: Vishal Chourasia <vishalc@linux.ibm.com>
Signed-off-by: Shivang Upadhyay <shivangu@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260511095055.82495-1-shivangu@linux.ibm.com
[harshpb: expanded comment to elobarate more on the rationale]
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
---
 hw/ppc/spapr_hcall.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
index 032805a8d0..60ba215e86 100644
--- a/hw/ppc/spapr_hcall.c
+++ b/hw/ppc/spapr_hcall.c
@@ -1105,6 +1105,15 @@ static target_ulong h_signal_sys_reset(PowerPCCPU *cpu,
                     continue;
                 }
             }
+
+            /* Skip quiesced CPUs - they are in RTAS stopped state and
+             * should not be reset. This prevents kdump hangs when CPUs
+             * are hotplugged but not yet started by the guest.
+             */
+            if (c->env.quiesced) {
+                continue;
+            }
+
             run_on_cpu(cs, spapr_do_system_reset_on_cpu, RUN_ON_CPU_NULL);
         }
         return H_SUCCESS;
-- 
2.52.0