question on s390x topology: KVM only, or also TCG?

[Claudio Fontana <cfontana@suse.de>]

Hello Pierre, Ilya,

I have a question on the s390x "topology" feature and examples.

Mainly, is this feature supposed to be KVM accelerator-only, or also available when using the TCG accelerator?

(docs/devel/s390-cpu-topology.rst vs https://www.qemu.org/docs/master/system/s390x/cpu-topology.html)

I see stsi-topology.c in target/s390x/kvm/ , so that part is clearly KVM-specific,

but in hw/s390x/cpu-topology.c I read:

"
 * - The first part in this file is taking care of all common functions                                                                     
 *   used by KVM and TCG to create and modify the topology.                                                                                 
 *                                                                                                                                          
 * - The second part, building the topology information data for the                                                                        
 *   guest with CPU and KVM specificity will be implemented inside                                                                          
 *   the target/s390/kvm sub tree.
"

In the docs/devel/s390-cpu-topology.rst

I see the example command:

 qemu-system-s390x \
    -enable-kvm \
    -cpu z14,ctop=on \
    -smp 1,drawers=3,books=3,sockets=2,cores=2,maxcpus=36 \
    -device z14-s390x-cpu,core-id=19,entitlement=high \
    -device z14-s390x-cpu,core-id=11,entitlement=low \
    -device z14-s390x-cpu,core-id=12,entitlement=high \
   ...


which uses KVM only.

In https://www.qemu.org/docs/master/system/s390x/cpu-topology.html

I read:

"Prerequisites:
To use the CPU topology, you need to run with KVM on a s390x host that uses the Linux kernel v6.0 or newer (which provide the so-called KVM_CAP_S390_CPU_TOPOLOGY capability that allows QEMU to signal the CPU topology facility via the so-called STFLE bit 11 to the VM).
"

So I would assume this is KVM-only, but then in the "Examples" section below I see the example:

"
$ qemu-system-s390x -m 2G \
  -cpu gen16b,ctop=on \
  -smp cpus=5,sockets=8,cores=4,maxcpus=32 \
  -device host-s390x-cpu,core-id=14 \
"

and

"
qemu-system-s390x -m 2G \
  -cpu gen16b,ctop=on \
  -smp cpus=1,sockets=8,cores=4,maxcpus=32 \
  \
  -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=1 \
  -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=2 \
  -device gen16b-s390x-cpu,drawer-id=1,book-id=1,socket-id=2,core-id=3 \
  \
  -device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=9 \
  -device gen16b-s390x-cpu,drawer-id=0,book-id=0,socket-id=0,core-id=14 \
  \
  -device gen16b-s390x-cpu,core-id=4,dedicated=on,entitlement=high
"

We received questions about this, so I hope you can shed some light, maybe it would be good to just update the web page to include -accel kvm or -enable-kvm everywhere for clarity?

Thanks for your help on this,

Claudio

-- 
Claudio Fontana
Engineering Manager Virtualization, SUSE Labs Core

SUSE Software Solutions Italy Srl