I have a platform with both LOCAL_APIC and LOCAL_X2APIC entries for
each CPU. However, the ids for the LOCAL_APIC entries are all
invalid ids of 255, so they have always been skipped in acpi_parse_lapic()
by this code from f3bf1dbe64b6 ("x86/acpi: Prevent LAPIC id 0xff from being
accounted"):
/* Ignore invalid ID */
if (processor->id == 0xff)
return 0;
With the change in this thread, the return value of 0 means that the
'count' variable in acpi_parse_entries_array() is incremented. The
positive return value means that 'has_lapic_cpus' is set, even though
no entries were actually matched. Then, when the MADT is iterated
with acpi_parse_x2apic(), the x2apic entries with ids less than 255
are skipped and most of my CPUs aren't recognized.
I think the original version of this change was okay for this case in
https://lore.kernel.org/lkml/87pm4bp54z.ffs@tglx/T/
P.S. I could be convinced that the MADT for my platform is somewhat
ill-formed and that I'm relying on pre-existing behavior. I'm not
well-versed enough in the topic to know for sure.