On Thu, Apr 27, 2017 at 02:51:31PM -0500, Michael Roth wrote: > Quoting David Gibson (2017-04-27 02:28:43) > > Migrating between different CPU versions is a bit complicated for ppc. > > A long time ago, we ensured identical CPU versions at either end by > > checking the PVR had the same value. However, this breaks under KVM > > HV, because we always have to use the host's PVR - it's not > > virtualized. That would mean we couldn't migrate between hosts with > > different PVRs, even if the CPUs are close enough to compatible in > > practice (sometimes identical cores with different surrounding logic > > have different PVRs, so this happens in practice quite often). > > > > So, we removed the PVR check, but instead checked that several flags > > indicating supported instructions matched. This turns out to be a bad > > idea, because those instruction masks are not architected information, but > > essentially a TCG implementation detail. So changes to qemu internal CPU > > modelling can break migration - this happened between qemu-2.6 and > > qemu-2.7. That was addressed by 146c11f1 "target-ppc: Allow eventual > > removal of old migration mistakes". > > > > Now, verification of CPU compatibility across a migration basically doesn't > > happen. We simply ignore the PVR of the incoming migration, and hope the > > cpu on the destination is close enough to work. > > > > Now that we've cleaned up handling of processor compatibility modes for > > pseries machine type, we can do better. We allow migration if: > > > > * The source and destination PVRs are for the same type of CPU, as > > determined by CPU class's pvr_match function > > OR * When the source was in a compatibility mode, and the destination CPU > > supports the same compatibility mode > > > > Signed-off-by: David Gibson <da...@gibson.dropbear.id.au> > > --- > > target/ppc/machine.c | 71 > > +++++++++++++++++++++++++++++++++++++++++++++++++--- > > 1 file changed, 68 insertions(+), 3 deletions(-) > > > > diff --git a/target/ppc/machine.c b/target/ppc/machine.c > > index 6cb3a48..20a46c9 100644 > > --- a/target/ppc/machine.c > > +++ b/target/ppc/machine.c > > @@ -8,6 +8,7 @@ > > #include "helper_regs.h" > > #include "mmu-hash64.h" > > #include "migration/cpu.h" > > +#include "qapi/error.h" > > > > static int cpu_load_old(QEMUFile *f, void *opaque, int version_id) > > { > > @@ -195,6 +196,30 @@ static void cpu_pre_save(void *opaque) > > } > > } > > > > +/* > > + * Determine if a given PVR is a "close enough" match to the CPU > > + * object. For TCG and KVM PR it would probably be sufficient to > > + * require an exact PVR match. However for KVM HV the user is > > + * restricted to a PVR exactly matching the host CPU. The correct way > > + * to handle this is to put the guest into an architected > > + * compatibility mode. However, to allow a more forgiving transition > > + * and migration from before this was widely done, we allow migration > > + * between sufficiently similar PVRs, as determined by the CPU class's > > + * pvr_match() hook. > > + */ > > +static bool pvr_match(PowerPCCPU *cpu, uint32_t pvr) > > +{ > > + PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu); > > + > > + if (pvr == pcc->pvr) { > > + return true; > > + } > > + if (pcc->pvr_match) { > > + return pcc->pvr_match(pcc, pvr); > > + } > > + return false; > > +} > > + > > static int cpu_post_load(void *opaque, int version_id) > > { > > PowerPCCPU *cpu = opaque; > > @@ -203,10 +228,31 @@ static int cpu_post_load(void *opaque, int version_id) > > target_ulong msr; > > > > /* > > - * We always ignore the source PVR. The user or management > > - * software has to take care of running QEMU in a compatible mode. > > + * If we're operating in compat mode, we should be ok as long as > > + * the destination supports the same compatiblity mode. > > + * > > + * Otherwise, however, we require that the destination has exactly > > + * the same CPU model as the source. > > */ > > - env->spr[SPR_PVR] = env->spr_cb[SPR_PVR].default_value; > > + > > +#if defined(TARGET_PPC64) > > + if (cpu->compat_pvr) { > > + Error *local_err = NULL; > > + > > + ppc_set_compat(cpu, cpu->compat_pvr, &local_err); > > + if (local_err) { > > + error_report_err(local_err); > > + error_free(local_err); > > + return -1; > > + } > > + } else > > +#endif > > + { > > + if (!pvr_match(cpu, env->spr[SPR_PVR])) { > > + return -1; > > + } > > + } > > + > > env->lr = env->spr[SPR_LR]; > > env->ctr = env->spr[SPR_CTR]; > > cpu_write_xer(env, env->spr[SPR_XER]); > > @@ -560,6 +606,24 @@ static const VMStateDescription vmstate_tlbmas = { > > } > > }; > > > > +static bool compat_needed(void *opaque) > > +{ > > + PowerPCCPU *cpu = opaque; > > + > > + return cpu->vhyp != NULL; > > +} > > Would it make sense to relax this to: > > cpu->vhyp != NULL && cpu->compat_pvr
Hm, so that's equivalent to just cpu->compat_pvr != 0, since compat_pvr should never be set when !vhyp. > ? This would at least allow cross-version migration in cases where we > weren't previously running in a compatibility mode. As it stands it > seems like this would rule out both new->old and old->new migration. Hrm. I thought ther section needed test just controlled whether the section was created on outgoing migration, not whether it was mandatory on an incoming migration. So old->new migration should I think work (well, no worse than it ever did). new->old would be broken by this though. So I guess I need some sort of compat flag so the older machine types don't generate this section. > Another possibility might be to only enable migration/checking of > compat_pvr for 2.10 and later, similar to the cpu_pre_2_8_migration > stuff. > > > + > > +static const VMStateDescription vmstate_compat = { > > + .name = "cpu/compat", > > + .version_id = 1, > > + .minimum_version_id = 1, > > + .needed = compat_needed, > > + .fields = (VMStateField[]) { > > + VMSTATE_UINT32(compat_pvr, PowerPCCPU), > > + VMSTATE_END_OF_LIST() > > + } > > +}; > > + > > const VMStateDescription vmstate_ppc_cpu = { > > .name = "cpu", > > .version_id = 5, > > @@ -613,6 +677,7 @@ const VMStateDescription vmstate_ppc_cpu = { > > &vmstate_tlb6xx, > > &vmstate_tlbemb, > > &vmstate_tlbmas, > > + &vmstate_compat, > > NULL > > } > > }; > -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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