Il 23/07/2013 19:41, Eduardo Habkost ha scritto: > On Tue, Jul 23, 2013 at 06:23:08PM +0200, Paolo Bonzini wrote: >> Il 23/07/2013 17:40, Eduardo Habkost ha scritto: >>> On Tue, Jul 23, 2013 at 05:09:02PM +0200, Paolo Bonzini wrote: >>>> Il 23/07/2013 16:13, Eduardo Habkost ha scritto: >>>>> On Tue, Jul 23, 2013 at 11:18:03AM +0200, Paolo Bonzini wrote: >>>>>> Il 22/07/2013 21:25, Eduardo Habkost ha scritto: >>>>>>> Bug description: QEMU currently gets all bits from GET_SUPPORTED_CPUID >>>>>>> for CPUID leaf 0xA and passes them directly to the guest. This makes >>>>>>> the guest ABI depend on host kernel and host CPU capabilities, and >>>>>>> breaks live migration if we migrate between host with different >>>>>>> capabilities (e.g. different number of PMU counters). >>>>>>> >>>>>>> This patch adds a "pmu-passthrough" property to X86CPU, and set it to >>>>>>> true only on "-cpu host", or on pc-*-1.5 and older machine-types. >>>>>> >>>>>> Can we just call the property "pmu"? It doesn't have to be passthough. >>>>> >>>>> Yes, but the only options we have today are "no PMU" and "passthrough >>>>> PMU". I wouldn't like to make "pmu=on" enable the passthrough behavior >>>>> implicitly (I don't want things that break live-migration to be enabled >>>>> without making it explicit that it is a host-dependent/passthrough >>>>> mode). >>>> >>>> I think "passthrough PMU" should be considered a bug except of course >>>> with "-cpu host". >>>> >>>> If "-cpu Nehalem,pmu=on" goes from passthrough to Nehalem-compatible in >>>> a future QEMU release, that'll be a bugfix. >>> >>> Exactly. But then I don't understand your suggestion. We still need a >>> property to enable pasthrough behavior on old machine-types (not >>> perfect, but a best-effort way to try to keep compatibility), >> >> Do we? >> >> We only need "pmu=on"---which right now is buggy on old machine types >> because it will always passthrough. > > I am not sure I understand what you are arguing for. > > You agree that pmu=on needs to keep the buggy passthrough behavior on > pc-1.5 and older, right?
I agree it needs to remain enabled on 1.5. But if, for example, 1.8 makes pmu=on emulate a Nehalem-compatible PMU, I think it is fine if pc-1.5 moves from a host-compatible PMU to a Nehalem-compatible PMU. The reason is that pc-1.5 has never guaranteed any feature of the emulated PMU. Paolo