[RFC PATCH 5/5] powerpc/smp: Remap boot CPU onto core 0 if >= nr_cpu_ids

[Michael Ellerman]

Jiri Bohac <jbohac at suse.cz> writes:
> On Tue, Jan 02, 2024 at 10:16:04AM +0530, Aneesh Kumar K.V wrote:
>> Michael Ellerman <mpe at ellerman.id.au> writes:
>> 
>> ....
>> 
>> >  #ifdef CONFIG_PPC64
>> >  int boot_cpu_hwid = -1;
>> > @@ -492,12 +493,26 @@ void __init smp_setup_cpu_maps(void)
>> >  			avail = !of_property_match_string(dn,
>> >  					"enable-method", "spin-table");
>> >  
>> > -		cpu = assign_threads(cpu, nthreads, avail, intserv);
>> > +		if (boot_core_hwid >= 0) {
>> > +			if (cpu == 0) {
>> > +				pr_info("Skipping CPU node %pOF to allow for boot core.\n", dn);
>> > +				cpu = nthreads;
>> > +				continue;
>> > +			}
>> >  
>> > -		if (cpu >= nr_cpu_ids) {
>> > +			if (be32_to_cpu(intserv[0]) == boot_core_hwid) {
>> > +				pr_info("Renumbered boot core %pOF to logical 0\n", dn);
>> > +				assign_threads(0, nthreads, avail, intserv);
>> > +				of_node_put(dn);
>> > +				break;
>> >
>> 
>> I was expecting a 'continue' here. Why 'break' the loop? The condition that
>> should break the loop should be cpu >= nr_cpu_ids 
>
> No, the patch seems correct:
>
> We're in the "if (boot_core_hwid >= 0)" branch, meaning that it
> was determined by early_init_dt_scan_cpus() that boot_cpuid >=
> nr_cpu_ids. So we loop until we get to the boot CPU, so it can be
> renumbered to 0. Once we do that we break, because we
> know we are already past nr_cpu_ids - otherwise boot_core_hwid
> would not be >= 0. 

Yes that's exactly right.

Thanks for answering for me (was on leave and still catching up).

cheers