LoongArch: Always enumerate MADT and setup logical-physical CPU mapping

Some drivers want to use cpu_logical_map(), early_cpu_to_node() and some
other CPU mapping APIs, even if we use "nr_cpus=1" to hard limit the CPU
number. This is strongly required for the multi-bridges machines.

Currently, we stop parsing the MADT if the nr_cpus limit is reached, but
to achieve the above goal we should always enumerate the MADT table and
setup logical-physical CPU mapping whether there is a nr_cpus limit.

Rework the MADT enumeration:

1. Define a flag "cpu_enumerated" to distinguish the first enumeration
   (cpu_enumerated=0) and the physical hotplug case (cpu_enumerated=1)
   for set_processor_mask().

2. If cpu_enumerated=0, stop parsing only when NR_CPUS limit is reached,
   so we can setup logical-physical CPU mapping; if cpu_enumerated=1,
   stop parsing when nr_cpu_ids limit is reached, so we can avoid some
   runtime bugs. Once logical-physical CPU mapping is setup, we will let
   cpu_enumerated=1.

3. Use find_first_zero_bit() instead of cpumask_next_zero() to find the
   next zero bit (free logical CPU id) in the cpu_present_mask, because
   cpumask_next_zero() will stop at nr_cpu_ids.

4. Only touch cpu_possible_mask if cpu_enumerated=0, this is in order to
   avoid some potential crashes, because cpu_possible_mask is marked as
   __ro_after_init.

5. In prefill_possible_map(), clear cpu_present_mask bits greater than
   nr_cpu_ids, in order to avoid a CPU be "present" but not "possible".

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

1 files changed, 3 insertions, 4 deletions

diff --git a/arch/loongarch/kernel/smp.c b/arch/loongarch/kernel/smp.c
index 1436d2465939..03b2b7669cf5 100644
--- a/arch/loongarch/kernel/smp.c
+++ b/arch/loongarch/kernel/smp.c
@@ -271,11 +271,10 @@ static void __init fdt_smp_setup(void)
 		if (cpuid >= nr_cpu_ids)
 			continue;
 
-		if (cpuid == loongson_sysconf.boot_cpu_id) {
+		if (cpuid == loongson_sysconf.boot_cpu_id)
 			cpu = 0;
-		} else {
-			cpu = cpumask_next_zero(-1, cpu_present_mask);
-		}
+		else
+			cpu = find_first_zero_bit(cpumask_bits(cpu_present_mask), NR_CPUS);
 
 		num_processors++;
 		set_cpu_possible(cpu, true);