Re: [libvirt] CPU models and feature probing (was Re: [Qemu-devel] [PATCH qom-cpu 00/16 v10] target-i386: convert CPU) features into properties

>> If there's no relation between a CPU model named, e.g., "Haswell" and >> the one on an Intel Haswell chip any more, then we should give them >> artificial names like "qemu64"; I strongly believe that Haswell >> definition in code should match that of the specs/hardware and if TCG >> can't emulate something that's one thing (subtractive: no AVX) and if >> KVM wants to speed up things that's another (additive: kvmvapic, >> in-kernel IRQ/PIT). What I am arguing against is watering the meaning of >> our definitions from "this is this model" to "this is handy". In >> particular, if we use the post_initialize hook like I suggested then >> -global is still able to override it and Eduardo's properties should >> correctly report them to libvirt. > > "Haswell" is named this way not only because it looks like Haswell, but > also because it has useful features you are going to find only on > Haswell, and you (probably) are not going to be able to run it on hosts > older than Haswell. That's the main real-world application of CPU > models: making sure the VMs can run on a specific subset of hosts. So, > if you choose "Haswell", you are telling the management stack "I know > this is going to run only on Haswell and newer CPUs". > > That's why x2apic is being proposed as an exception: it can be enabled > on any host, because it doesn't depend on host-side support. That's why > I propose we enable it on CPU models that don't have x2apic in the real > world. > > (BTW, what is the relation between this subject and static properties? I > was expecting this to be discussed in the other existing thread about > x2apic) Sorry if I replied to two different series at once, that was for "Enabling x2apic by default in KvM (was Re: [Qemu-devel] [PATCH] target-i386: enable x2apic by default on more recent CPU models)". Which is connected to CPU models/subclasses in what those types are supposed to be good for if we define them. Let's take an obvious example. Jan wants to emulate a legacy isapc machine with -cpu 486. Then it feels counterproductive to enable the latest and greatest features such a machine never had. If the user wants to have the latest and greatest features, she can choose a newer -cpu model which already has the flag today.