lib/s390x: fix SMP setup bug

The lowcore pointer pointing to the current CPU (THIS_CPU) was not
initialized for the boot CPU. The pointer is needed for correct
interrupt handling, which is needed in the setup process before the
struct cpu array is allocated.

The bug went unnoticed because some environments (like qemu/KVM) clear
all memory and don't write anything in the lowcore area before starting
the payload. The pointer thus pointed to 0, an area of memory also not
used. Other environments will write to memory before starting the
payload, causing the unit tests to crash at the first interrupt.

Fix by assigning a temporary struct cpu before the rest of the setup
process, and assigning the pointer to the correct allocated struct
during smp initialization.

Fixes: 4e5dd758 ("lib: s390x: better smp interrupt checks")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Tested-by: Nico Boehr <nrb@linux.ibm.com>
Reported-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20220818152114.213135-1-imbrenda@linux.ibm.com


Message-Id: <20220818152114.213135-1-imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>