Re: [libvirt PATCH 0/9] [RFC] Dynamic CPU models

libvirt and qemu cpu models are out of sync. libvirt cpu models are considered static and never changing, whereas qemu cpu models have changed over time. This leads to problems and confusion for users, as the cpu models they specify may have different features than what they expect. This series introduces two new APIs to enumerate and retrieve cpu models dynamically from the hypervisor at runtime, allowing the user to make use of cpu models from newer hypervisor versions even if libvirt is not aware of them. These two new functions are intended as building blocks onto which higher-level functionality can be build, e.g. the ability to specify a hypervisor cpu model directly in a domain xml. With these two functions alone this has to be done manually for now, i.e. enumerate the available cpu models, retrieve the cpu description and manually insert it into the domain xml accordingly:

$ virsh hypervisor-cpu-models --arch x86_64 Name Alias ---------------------------------------------------- max host base qemu64-v1 qemu64 qemu64-v1 qemu32-v1 qemu32 qemu32-v1 phenom-v1 phenom phenom-v1 (...) $ virsh hypervisor-cpu-definition --machine pc --arch x86_64 qemu64 <cpu> <model>qemu64</model> <feature name='cmov'/> <feature name='mmx'/> <feature name='xd'/> <feature name='x-intel-pt-auto-level'/> <feature name='kvm_asyncpf'/> <feature name='kvm-asyncpf'/> <feature name='legacy-cache'/> <feature name='vmware-cpuid-freq'/> <feature name='mce'/> <feature name='mca'/> <feature name='msr'/> <feature name='fxsr'/> <feature name='cpuid-0xb'/> <feature name='kvm_pv_eoi'/> <feature name='pni'/> <feature name='x2apic'/> <feature name='i64'/> <feature name='pae'/> <feature name='pat'/> <feature name='sse'/> <feature name='kvm_nopiodelay'/> <feature name='kvm-nopiodelay'/> <feature name='kvmclock-stable-bit'/> <feature name='hypervisor'/> <feature name='syscall'/> <feature name='x-migrate-smi-count'/> <feature name='full-cpuid-auto-level'/> <feature name='sse3'/> <feature name='sse2'/> <feature name='kvm-pv-eoi'/> <feature name='cx8'/> <feature name='pge'/> <feature name='fill-mtrr-mask'/> <feature name='cx16'/> <feature name='de'/> <feature name='clflush'/> <feature name='tsc'/> <feature name='fpu'/> <feature name='check'/> <feature name='apic'/> <feature name='kvm-steal-time'/> <feature name='kvm_steal_time'/> <feature name='kvmclock'/> <feature name='l3-cache'/> <feature name='nx'/> <feature name='tcg-cpuid'/> <feature name='lm'/> <feature name='pse'/> <feature name='sep'/> <feature name='kvm'/> <feature name='lahf-lm'/> <feature name='lahf_lm'/> <feature name='mtrr'/> <feature name='pse36'/> </cpu> The returned cpu description is currently completely unfiltered, as can be seen by the duplicate entries differing only in "-" vs. "_" usage, inclusion of experimental feature flags and generally, flags that are not recognized by libvirt. One possibility to adress this would be to extend virQEMUCapsCPUFeatureTranslate.

Before I continue with this, I would love to hear other people's thoughts, comments and potential use cases.