Re: [libvirt] CPU type/flags when converting a physical machine to run on libvirt

On Fri, May 02, 2014 at 01:07:05PM +0100, Richard W.M. Jones wrote: > - Should we try to reflect the CPU type of the physical machine in > the virtual machine? eg. If it's an Opteron, we generate an > Opteron target machine. (I believe the answer is *no*, because > this is not live migration, and most guests can boot on any > compatible CPU). I see no reason to _not_ choose Opteron_Gx, if you know the host CPU is always going to be an Opteron_Gx.

If the user simply plans to convert an existing physical machine to a single-VM machine and has no plans to ever migrate the VM, it makes sense to use "-cpu host" (but beware: this may uncover a few QEMU bugs). if the user plans to migrate the VM to a _similar_ host later, it makes sense to use an existing CPU model name that matches the host CPU (see "host-cpu-model" below). If the user plans to migrate the VM to a very different host later it makes sense to be more conservative and simply use the default CPU model. In other words: I don't know what's a good default because I don't know your use case very well. > > - How can I ask libvirt to give me the best possible CPU, and not > some baseline? Normally I use host-model, but I think that > prevents migration. The best possible CPU is "-cpu host" (host-passthrough in libvirt), but that doesn't allow migration. This may uncover QEMU bugs (but it is much better today than it was 1 or 2 years ago). The best possible CPU which allows migration is the one you get when explicitly asking libvirt to expand host-model (including baseline + flags). This is likely to uncover QEMU and libvirt bugs. A safer option is to use only the base CPU model (not the additional flags) provided by libvirt when asking about the host CPU model (I believe this is called "host-cpu-model" on virt-manager code).

> - What CPU flags should be reflected in the target libvirt XML? > > - Is it worth modelling CPU thread layout? (I suspect this will be a > lot of work with the potential to break things rather than provide > any measurable benefits.) I wouldn't recommend this unless: 1) you know the VM will be kept running in the same host or on a similar host; 2) you pin the VCPUs/threads to corresponding host CPUs/threads. This may also uncover QEMU bugs, so I wouldn't do this by default unless the user explicitly asks for it.

> - Is there anything else I haven't thought about? In the future you may want to support multi-node NUMA VMs. This is similar to the multi-core/multi-thread case: it makes sense if you know the VM is going to run on a host with similar topology, and if you manually pin the guest nodes to the host nodes (something which is not possible yet, but should be possible in the near future).