Notes on BAD_APICID, Was: [PATCH 0/3] x86/apic: Misc pruning

[Andrew Cooper]

On 02/11/2023 12:26 pm, Andrew Cooper wrote:
> Seriously, this work started out trying to fix a buggy comment.  It
> escalated somewhat...  Perform some simple tidying.

Another dodgy construct spotted while doing this work is

#ifdef CONFIG_X86_32
 #define BAD_APICID 0xFFu
#else
 #define BAD_APICID 0xFFFFu
#endif

considering that both of those "bad" values are legal APIC IDs in an
x2APIC system.

The majority use is as a sentential (of varying types - int, u16
mostly), although the uses for NUM_APIC_CLUSTERS, and
safe_smp_processor_id() look suspect.

In particular, safe_smp_processor_id() *will* malfunction on some legal
CPUs, and needs to use -1 (32 bits wide) to spot the intended error case
of a bad xAPIC mapping.

However, it's use in amd_pmu_cpu_starting() from topology_die_id() looks
broken.  Partly because the error handling is (only) a WARN_ON_ONCE(),
and also because nb->nb_id's sentinel value is -1 of type int.

I suspect there's a lot of cleaning to be done here too.

~Andrew