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

[Shivang Upadhyay <shivangu@linux.ibm.com>]

Hi Shrikanth, Thanks for your review.

On Mon, 2026-03-30 at 14:19 +0530, Shrikanth Hegde wrote:
> Hi Shivang.
> 
> Few nitpicks.
> 
> On 3/30/26 11:52 AM, Shivang Upadhyay wrote:
> > 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.
> > 
> > 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();
> >   	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;
> >   
> > +
> > +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;
> 
> Please follow inverted christmas tree for variables.
Ack.
> 
> 
> Can the below block be re-written with less indentations?
> One level indentation removal could be.
> 
> if (cpu == NMI_IPI_ALL_OTHERS && crash_nmi_ipi)
> 	hwcpu = <>
> else if (cpu == NMI_IPI_ALL_OTHERS)
> 	hwcpu = H_SIGNAL_SYS_RESET_ALL_OTHERS;
> else
> 	<existing remaining logic>
> 
> >   
> > +	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) {
> 
> Maybe below to reduce one more indentation level.
> 
> if ( k == curcpu)
> 	continue;
> 
Sure, i can rewrite it that way.
> 
> > +					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) {
> 
> It either 1 or 0. So if(paca_ptrs[hwcpu]->cpu_start) is enough.
Ack.

~Shivang.