Re: [PATCH v2] pseries/kexec: skip resetting CPUs added by firmware but not started by the kernel

Shivang Upadhyay
2026-03-31T06:44:35Z

Hi Srikar,

Thanks for reviewing.

On Tue, 2026-03-31 at 10:03 +0530, Srikar Dronamraju wrote:
> * Shivang Upadhyay <shivangu@linux.ibm.com> [2026-03-30 11:52:06]:
>
> > During DLPAR operations, the newly added CPUs start in halted mode.
> > The kernel then takes some time to initialize those CPUs internally
> > and
> > start them using the "start-cpu" RTAS call. However, if a kexec
> > crash
> > occurs in this window (before the new CPU has been initialized),
> > the kexec NMI will try to reset all other CPUs from the crashing
> > CPU.
> > This leads to firmware starting the uninitialized CPUs as well.
>
> What would happen if an non kexec crash nmi was delivered when we did
> a
> DLPAR operation and before the CPUs are initialized?

As per my understanding, similar hang like situation should happen
there also. But I think this case would be highly unlikely. Crash's
case is little special, because this is called for offline cpus also.

>
> >
> > This can cause the kdump kernel to hang during bring-up.
> >
> > Sample Log:
> >   [175993.028231][ T1502] NIP [00007fffb953f394] 0x7fffb953f394
> >   [175993.028314][ T1502] LR [00007fffb953f394] 0x7fffb953f394
> >   [175993.028390][ T1502] --- interrupt: 3000
> >   [    5.519483][    T1] Processor 0 is stuck.
> >   [   11.089481][    T1] Processor 1 is stuck.
> >
> > To fix this, only issue the system-reset hcall to CPUs that have
> > actually been started by the kernel.
> >
> > Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> > Cc: Srikar Dronamraju <srikar@linux.ibm.com>
> > Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
> > Cc: Nysal Jan K.A. <nysal@linux.ibm.com>
> > Cc: Vishal Chourasia <vishalc@linux.ibm.com>
> > Cc: Ritesh Harjani <ritesh.list@gmail.com>
> > Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
> > Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
> > Signed-off-by: Shivang Upadhyay <shivangu@linux.ibm.com>
> > ---
> > Changelog:
> >
> > V2:
> >   * added set_crash_nmi_ipi to saperate crash's case from other
> > nmi_ipi
> >     users
> >
> > V1:
> >   *
> > https://lore.kernel.org/all/20251205142825.44698-1-shivangu@linux.ibm.com/
> > ---
> >  arch/powerpc/include/asm/smp.h       |  1 +
> >  arch/powerpc/kernel/smp.c            |  1 +
> >  arch/powerpc/platforms/pseries/smp.c | 29
> > +++++++++++++++++++++++++++-
> >  3 files changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/include/asm/smp.h
> > b/arch/powerpc/include/asm/smp.h
> > index e41b9ea42122..cb74201f5674 100644
> > --- a/arch/powerpc/include/asm/smp.h
> > +++ b/arch/powerpc/include/asm/smp.h
> > @@ -47,6 +47,7 @@ struct smp_ops_t {
> >   void  (*cause_ipi)(int cpu);
> >  #endif
> >   int   (*cause_nmi_ipi)(int cpu);
> > + void  (*set_crash_nmi_ipi)(void);
> >   void  (*probe)(void);
> >   int   (*kick_cpu)(int nr);
> >   int   (*prepare_cpu)(int nr);
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 3467f86fd78f..3390ee8adf79 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -594,6 +594,7 @@ void crash_send_ipi(void
> > (*crash_ipi_callback)(struct pt_regs *))
> >  {
> >   int cpu;
> >
> > + smp_ops->set_crash_nmi_ipi();
>
> Shouldn't we be checking got smp_ops->set_crash_nmi_ipi being non-
> NULL
> before calling. Dont we expect set_crash_nmi_ipi() to be NULL in
> PowerNV
> after this patch? Or what would happen if set_crash_nmi_ipi() was
> called in
> PowerNV code?
>

Yeah, that should be a Bug, Ill fix this in new patch. Thanks for
catching this.

> >   smp_send_nmi_ipi(NMI_IPI_ALL_OTHERS, crash_ipi_callback,
> > 1000000);
> >   if (kdump_in_progress() && crash_wake_offline) {
> >   for_each_present_cpu(cpu) {
> > diff --git a/arch/powerpc/platforms/pseries/smp.c
> > b/arch/powerpc/platforms/pseries/smp.c
> > index db99725e752b..c6c2baacca9a 100644
> > --- a/arch/powerpc/platforms/pseries/smp.c
> > +++ b/arch/powerpc/platforms/pseries/smp.c
> > @@ -51,6 +51,9 @@
> >   */
> >  static cpumask_var_t of_spin_mask;
> >
> > +
>
> Nit: Are we adding an unnecessary newline?

Ack.

>
> > +static int crash_nmi_ipi;
> > +
> >  /* Query where a cpu is now.  Return codes #defined in
> > plpar_wrappers.h */
> >  int smp_query_cpu_stopped(unsigned int pcpu)
> >  {
> > @@ -171,12 +174,35 @@ static void dbell_or_ic_cause_ipi(int cpu)
> >   ic_cause_ipi(cpu);
> >  }
> >
> > +static void pseries_set_crash_nmi_ipi(void)
> > +{
> > + crash_nmi_ipi = 1;
> > +}
> > +
> >  static int pseries_cause_nmi_ipi(int cpu)
> >  {
> >   int hwcpu;
> > + int k, curcpu;
> >
> > + curcpu = smp_processor_id();
> >   if (cpu == NMI_IPI_ALL_OTHERS) {
> > - hwcpu = H_SIGNAL_SYS_RESET_ALL_OTHERS;
> > + if (crash_nmi_ipi) {
> > + for_each_present_cpu(k) {
> > + if (k != curcpu) {
> > + hwcpu =
> > get_hard_smp_processor_id(k);
> > +
> > + /* it is possible that cpu
> > is present,
> > + * but not started yet.
> > + */
> > +
> > + if (paca_ptrs[hwcpu]-
> > >cpu_start == 1) {
> > + plpar_signal_sys_r
> > eset(hwcpu);
> > + }
> > + }
> > + }
> > + return 1;
> > + } else
> > + hwcpu = H_SIGNAL_SYS_RESET_ALL_OTHERS;
> >   } else {
> >   if (cpu < 0) {
> >   WARN_ONCE(true, "incorrect cpu parameter
> > %d", cpu);
> > @@ -243,6 +269,7 @@ static struct smp_ops_t pseries_smp_ops = {
> >   .message_pass = NULL, /* Use
> > smp_muxed_ipi_message_pass */
> >   .cause_ipi = NULL, /* Filled at runtime by
> > pSeries_smp_probe() */
> >   .cause_nmi_ipi = pseries_cause_nmi_ipi,
> > + .set_crash_nmi_ipi = pseries_set_crash_nmi_ipi,
> >   .probe = pSeries_smp_probe,
> >   .prepare_cpu = pseries_smp_prepare_cpu,
> >   .kick_cpu = smp_pSeries_kick_cpu,
> > --
> > 2.53.0
> >

Thanks

~Shivang.