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? In that case, how do you suggest I let QEMU know that only pc-1.5 and older should have the buggy behavior enabled when pmu=on? I understand that compat_props is the appropriate place for that, and that's why I need a "please-enable-the-old-buggy-pmu-passthrough-behavior" property that I can add to PC_COMPAT_1_5. > > Paolo > > > and I > > named that option "pmu-passthrough". How do you think we should name it? > -- Eduardo