libperf: Document struct perf_cpu int16_t ABI limitation

struct perf_cpu.cpu is int16_t, limiting perf to 32767 CPUs.  This is
part of the libperf ABI (returned by value from perf_cpu_map__cpu() and
friends), so widening it requires an ABI bump.

Add a comment on the struct definition noting this, and create a TODO
file to collect future ABI changes so they can be batched into a single
version bump.

Cc: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/TODO

@@ -0,0 +1,22 @@
+Future ABI changes
+==================
+
+This file collects items that require a libperf ABI bump.  Each entry
+should describe the current limitation, the desired end state, and the
+scope of the change so that a future ABI revision can batch them
+together.
+
+1. Widen struct perf_cpu.cpu from int16_t to int
+   - Current limit: 32767 CPUs.  No architecture exceeds this today
+     (x86_64 max is 8192, arm64 is 4096), but NR_CPUS limits keep
+     growing.  perf clamps to INT16_MAX in set_max_cpu_num() as a
+     safety net.
+   - Scope: struct perf_cpu is embedded everywhere — perf_cpu_map__cpu(),
+     perf_cpu_map__min(), perf_cpu_map__max(), perf_cpu_map__has(), the
+     for_each_cpu macros, and all internal callers.  The perf_cpu_map
+     internal array (RC_CHK_ACCESS(map)->map[]) stores struct perf_cpu
+     directly.  Widening changes the struct layout and every function
+     that returns or accepts struct perf_cpu by value.
+   - Migration: bump LIBPERF version in libperf.map, audit all
+     sizeof(struct perf_cpu) assumptions, update perf.data
+     serialization if needed.

tools/lib/perf/include/perf/cpumap.h

@@ -6,7 +6,13 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-/** A wrapper around a CPU to avoid confusion with the perf_cpu_map's map's indices. */
+/**
+ * struct perf_cpu - wrapper around a CPU number.
+ * @cpu: CPU number, -1 for the "any CPU"/dummy value.
+ *
+ * int16_t limits this to 32767 CPUs.  Widening to int requires a libperf
+ * ABI bump — see tools/lib/perf/TODO for the full scope.
+ */
 struct perf_cpu {
 	int16_t cpu;
 };