Daniel reported that with Commit 4ca234a9 ("powerpc/smp: Stop
updating cpu_core_mask") QEMU was unable to set single NUMA node SMP
topologies such as:
 -smp 8,maxcpus=8,cores=2,threads=2,sockets=2
 i.e he expected 2 sockets in one NUMA node.

The above commit helped to reduce boot time on Large Systems for
example 4096 vCPU single socket QEMU instance. PAPR is silent on
having more than one socket within a NUMA node.

cpu_core_mask and cpu_cpu_mask for any CPU would be same unless the
number of sockets is different from the number of NUMA nodes.

One option is to reintroduce cpu_core_mask but use a slightly
different method to arrive at the cpu_core_mask. Previously each CPU's
chip-id would be compared with all other CPU's chip-id to verify if
both the CPUs were related at the chip level. Now if a CPU 'A' is
found related / (unrelated) to another CPU 'B', all the thread
siblings of 'A' and thread siblings of 'B' are automatically marked as
related / (unrelated).

Also if a platform doesn't support ibm,chip-id property, i.e its
cpu_to_chip_id returns -1, cpu_core_map holds a copy of
cpu_cpu_mask().

Fixes: 4ca234a9 ("powerpc/smp: Stop updating cpu_core_mask")
Reported-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210415120934.232271-2-srikar@linux.vnet.ibm.com