Hi Brice,
Thank you for your suggestions!

Patches #4 and #5 are changing the meaning the core_siblings (in the
past, it always returned all threads in the entire package). All
existing user-space tools will see each die as a separate package until
they are updated to read die_siblings too. It only matters for multi-die
CPUs when running a recent kernel with an old userspace tool, but it may
still be consider as a sysfs ABI change.

I agree.

Exhibit 1 is the "lscpu" program.

Worse, things will break again if you ever add tile_siblings for
CPUID.1f "Tiles". User-space will suddenly see 2 dies of 2 cores instead
1 die of 2 tiles of 2 cores.

Agreed, the existing naming scheme is not resilient to future additions.

I understand that this isn't easy to fix. But I want to make sure people
are aware of the meaning of this change.

Here is my list of applications that care about the new CPUID leaf
and the concepts of packages and die:

cpuid
lscpu
x86_energy_perf_policy
turbostat

The proper way to avoid this is to stop having file foo_siblings refer
to "the container of foo" instead of "foo itself" (because that
container changes when you add intermediate levels). Rename sysfs files
like below, and you don't get any breakage anymore when adding
intermediate levels:

thread_siblings -> core_threads (can we do sysfs alias or symlink to
keep the old name?)

core_siblings -> die_threads

die_siblings -> package_threads (needs an alias too)

The documentation would also be much easier to read since "die_threads"
is obviously "human-readable list of cpuX's hardware threads within the
same die_id". And no need to modify the doc anymore when adding levels :)

I like your idea!

Hm, I think i'd skip creating "die_siblings", as it adds to the
fragile legacy naming scheme
that we want to deprecate.

And although it is ill-defined and has a mis-leading name, I now think
it would be
better to leave "core_siblings" as defined -- a legacy synonym for
"package_threads".  Deprecate it, but keep its original definition
until it is removed.

Updated applications would use:

core_threads
die_threads
package_threads

and they'll be future proof if/when we add any new levels.

the legacy thread_siblings and core_siblings will stick around as aliases:

core_threads (thread_siblings)
die_threads
package_threads (core_siblings)

thanks!
Len Brown, Intel Open Source Technology Center