Re: [libvirt] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines

On 10/25/2017 12:23 PM, David Hildenbrand wrote: David, I disagree if your proposal is to generally tolerate new cpu features in old machine types. This *might* work for gs but how do you guaranty that guests do not behave differently/wrong when suddenly new cpu features are made available to guest when (re-)starting them. That is my feedback for the qemu side of this mater. Regarding the libvirt side of this: When looking at https://libvirt.org/formatdomain.html#elementsCPU I found the following sentence: Since the CPU definition is copied just before starting a domain, exactly the same XML can be used on different hosts while still providing the best guest CPU each host supports. My interpretation of "the best guest CPU each host supports" is that besides limiting factors like hardware, kernel and qemu capabilities the requested machine type for the guest is a limiting factor as well. Nevertheless if my interpretation is found to be incorrect than we should think about another new cpu mode that includes the machine type into the "best guest CPU" detection. My assumption is that we must not require the users to know which cpu model they manually need to define to match a specific machine type AND we want to guarantee that guests run without risking any side effects by tolerating any additional cpu features. -- Mit freundlichen Grüßen/Kind regards Boris Fiuczynski IBM Deutschland Research & Development GmbH Vorsitzender des Aufsichtsrats: Martina Köderitz Geschäftsführung: Dirk Wittkopp Sitz der Gesellschaft: Böblingen Registergericht: Amtsgericht Stuttgart, HRB 243294