In panic path, fadump is triggered via a panic notifier function.
Before calling panic notifier functions, smp_send_stop() gets called,
which stops all CPUs except the panic'ing CPU. Commit 8389b37d
("powerpc: stop_this_cpu: remove the cpu from the online map.") and
again commit bab26238 ("powerpc: Offline CPU in stop_this_cpu()")
started marking CPUs as offline while stopping them. So, if a kernel
has either of the above commits, vmcore captured with fadump via panic
path would not process register data for all CPUs except the panic'ing
CPU. Sample output of crash-utility with such vmcore:

  # crash vmlinux vmcore
  ...
        KERNEL: vmlinux
      DUMPFILE: vmcore  [PARTIAL DUMP]
          CPUS: 1
          DATE: Wed Nov 10 09:56:34 EST 2021
        UPTIME: 00:00:42
  LOAD AVERAGE: 2.27, 0.69, 0.24
         TASKS: 183
      NODENAME: XXXXXXXXX
       RELEASE: 5.15.0+
       VERSION: #974 SMP Wed Nov 10 04:18:19 CST 2021
       MACHINE: ppc64le  (2500 Mhz)
        MEMORY: 8 GB
         PANIC: "Kernel panic - not syncing: sysrq triggered crash"
           PID: 3394
       COMMAND: "bash"
          TASK: c0000000150a5f80  [THREAD_INFO: c0000000150a5f80]
           CPU: 1
         STATE: TASK_RUNNING (PANIC)

  crash> p -x __cpu_online_mask
  __cpu_online_mask = $1 = {
    bits = {0x2, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
  }
  crash>
  crash>
  crash> p -x __cpu_active_mask
  __cpu_active_mask = $2 = {
    bits = {0xff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
  }
  crash>

While this has been the case since fadump was introduced, the issue
was not identified for two probable reasons:

  - In general, the bulk of the vmcores analyzed were from crash
    due to exception.

  - The above did change since commit 8341f2f2 ("sysrq: Use
    panic() to force a crash") started using panic() instead of
    deferencing NULL pointer to force a kernel crash. But then
    commit de6e5d38 ("powerpc: smp_send_stop do not offline
    stopped CPUs") stopped marking CPUs as offline till kernel
    commit bab26238 ("powerpc: Offline CPU in stop_this_cpu()")
    reverted that change.

To ensure post processing register data of all other CPUs happens
as intended, let panic() function take the crash friendly path (read
crash_smp_send_stop()) with the help of crash_kexec_post_notifiers
option. Also, as register data for all CPUs is captured by f/w, skip
IPI callbacks here for fadump, to avoid any complications in finding
the right backtraces.
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211207103719.91117-2-hbathini@linux.ibm.com