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

[Thomas Huth <thuth@redhat.com>]

On 14/03/2024 16.49, Claudio Fontana wrote:
> 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?

  Hi Claudio!

Pierre left IBM, please CC: Nina with regards to s390x topology instead.

But with regards to your question, I think I can answer that, too: The 
topology feature is currently working with KVM only, yes. It hasn't been 
implemented for TCG yet.

> (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?

Yes, it would be better to include "-accel kvm" in those examples. Would you 
like to send a patch?

  Thanks,
   Thomas