On 25.10.19 04:25, Eduardo Habkost wrote:
We had introduced versioned CPU models in QEMU 4.1, including a
method for querying for CPU model versions using
Interesting, I was not aware of that.
On s390x, we somewhat have versioned CPU models, but we decided against giving them explicit names (e.g., z13-v1 or z13-4.1.0),
because it didn't really seem to be necessary. (and we often implement/add features for older CPU models, there is a lot of
fluctuation) Actually, you would have had to add "z13-z/VM-x.x.x" or "z13-LPAR-x.x.x" or
"z13-KVM-x.x.x" to model the features you actually see in all the different virtual environments ("what a CPU
looks like"). Not to talk about QEMU versions in addition to that. So we decided to always model what you would see under
LPAR and are able to specify for a KVM guest. So you can use "z13" in an up-to-date LPAR environment, but not in a z/VM
environment (you would have to disable features).
Each (!base) CPU model has a specific feature set per machine. Libvirt uses
query-cpu-model-expansion() to convert this model+machine to a machine-independent
representation. That representation is sufficient for all use cases we were aware of
(esp. "virsh domcapabilities", the host CPU model, migration).
While s390x has versioned CPU models, we have no explicit way of specifying them for
older machines, besides going over query-cpu-model-expansion and using expanded
"base model + features".
I can see that this might make sense on x86-64, where you only have maybe 3 versions of a CPU (e.g., the one Intel
messed up first - Haswell, the one Intel messed up next - Haswell-noTSX, and the one that Intel eventually did right -
Haswell-noTSX-IBRS) and you might want to specify "Haswell" vs. "Haswell-IBRS" vs.
"Haswell-noTSX-IBRS". But actually, you will always want to go for "Haswell-noTSX-IBRS", because
you can expect to run in updated environments if I am not wrong, everything else are corner cases.
Of course, versioned CPU model are neat to avoid "base model + list of
features", but at least for expanding the host model on s390x, it is not really
helpful. When migrating, the model expansion does the trick.
I haven't looked into details of "how to specify or model versions". Maybe IBM
wants to look into creating versions for all the old models we had. But again, not sure
if that is of any help for s390x. CCing IBM.