[Xen-users] CPU Topology Incorrect on AMD CPUs using NUMA Nodes

[Jarrah Gosbell <Jarrah.Gosbell@xxxxxxxxxxxxxxxxxxxxxxxx>]

Good Morning,

I have been trying to debug why my Threadripper 2950X based system
reports an incorrect number of CPUs under Xen and am looking for some
direction on how to fix the problem.

Due to the recent list of CPU bugs, I have SMT disabled on my system,
which results in Xen reporting 8 available CPUs rather than the 16 found
in my processor. With SMT enabled, Xen reports 32 threads, but in an
incorrect topology.

This is evident from the following `xl info` output:

---

host                   : dom0
release                : 5.5.9-1.qubes.x86_64
version                : #1 SMP Sun Mar 15 07:00:08 CET 2020
machine                : x86_64
nr_cpus                : 32
max_cpu_id             : 127
nr_nodes               : 2
cores_per_socket       : 8
threads_per_core       : 4
---

I believe this may have something to do with Xen seeing cores on
different nodes as the same physical CPU. This was determined using the
following `xl info -n` output:

---

cpu_topology           :
cpu:    core    socket     node
  0:       0        0        0
  1:       0        0        0
<snip>
 16:       0        0        1
 17:       0        0        1
<snip>

---

This would result in having 8 physical cores, 1 socket and 4 threads per
core, as above.


I have attempted the following in the debugging process:

* Changed BIOS configuration (memory access mode). Result was 8 cores, 2
threads per core.

* Tested Xen versions 4.8, 4.11 and 4.13 between 2 different dom0
distros (Qubes OS/Fedora and Debian). Result as above.

* Tested multiple configurations of SMT on/off in Xen/Bios. Result was
either as above, or 8 cores, 1 thread per core.

I've previously been working with Qubes OS. But further testing on
Debian today has made it obvious that this bug is not Qubes OS related.
Qubes specific detail can be found here:
https://github.com/qubesos/qubes-issues/issues/5743

Thanks for any help and pointers.
Jarrah.


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