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Re: [Qemu-ppc] [PATCHv3 4/4] ppc: Rework CPU compatibility testing acros
From: |
Greg Kurz |
Subject: |
Re: [Qemu-ppc] [PATCHv3 4/4] ppc: Rework CPU compatibility testing across migration |
Date: |
Mon, 29 May 2017 12:51:59 +0200 |
On Fri, 26 May 2017 14:16:30 +1000
David Gibson <address@hidden> wrote:
> On Thu, May 04, 2017 at 12:07:47PM +0200, Greg Kurz wrote:
> > On Thu, 27 Apr 2017 17:28:43 +1000
> > David Gibson <address@hidden> wrote:
> >
> > > 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 <address@hidden>
> > > ---
> > > 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);
> >
> > As already mentioned during the review of RFCv2, this calls
> > cpu_synchronize_state(CPU(cpu)) and trashes the registers.
> >
> > The following changes avoid that:
>
> This is a really ugly fix, and I think it misses the point.
>
> If a synchronize_state() trashes state here, it means we've already
> altered register state while not synchronized, which is a pre-existing
> bug.
>
This is exactly what happens when processing an incoming migration:
1) reset the cpu (clears the cpu dirty flags)
2) "alter register state" according to the migration stream
3) synchronize all registers with cpu_synchronize_all_post_init()
So I'm not sure where we have a pre-existing bug... or maybe document
that cpu_synchronize_state() shouldn't be called when processing
incoming migration (and why should it be since we synchronize all
registers at the end?).
> >
> > --- a/target/ppc/compat.c
> > +++ b/target/ppc/compat.c
> > @@ -118,7 +118,8 @@ bool ppc_check_compat(PowerPCCPU *cpu, uint32_t
> > compat_pvr,
> > return true;
> > }
> >
> > -void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, Error **errp)
> > +void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, bool sync_needed,
> > + Error **errp)
> > {
> > const CompatInfo *compat = compat_by_pvr(compat_pvr);
> > CPUPPCState *env = &cpu->env;
> > @@ -138,7 +139,9 @@ void ppc_set_compat(PowerPCCPU *cpu, uint32_t
> > compat_pvr, Error **errp)
> > pcr = compat->pcr;
> > }
> >
> > - cpu_synchronize_state(CPU(cpu));
> > + if (sync_needed) {
> > + cpu_synchronize_state(CPU(cpu));
> > + }
> >
> > cpu->compat_pvr = compat_pvr;
> > env->spr[SPR_PCR] = pcr & pcc->pcr_mask;
> > @@ -162,7 +165,7 @@ static void do_set_compat(CPUState *cs, run_on_cpu_data
> > arg)
> > PowerPCCPU *cpu = POWERPC_CPU(cs);
> > SetCompatState *s = arg.host_ptr;
> >
> > - ppc_set_compat(cpu, s->compat_pvr, &s->err);
> > + ppc_set_compat(cpu, s->compat_pvr, true, &s->err);
> > }
> >
> > void ppc_set_compat_all(uint32_t compat_pvr, Error **errp)
> > diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
> > index 1d8f2fcd4a46..057785347820 100644
> > --- a/target/ppc/cpu.h
> > +++ b/target/ppc/cpu.h
> > @@ -1364,7 +1364,8 @@ static inline int cpu_mmu_index (CPUPPCState *env,
> > bool ifetch)
> > #if defined(TARGET_PPC64)
> > bool ppc_check_compat(PowerPCCPU *cpu, uint32_t compat_pvr,
> > uint32_t min_compat_pvr, uint32_t max_compat_pvr);
> > -void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, Error **errp);
> > +void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, bool sync_needed,
> > + Error **errp);
> > #if !defined(CONFIG_USER_ONLY)
> > void ppc_set_compat_all(uint32_t compat_pvr, Error **errp);
> > #endif
> > diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> > index 20a46c95a596..fda63532b041 100644
> > --- a/target/ppc/machine.c
> > +++ b/target/ppc/machine.c
> > @@ -239,7 +239,7 @@ static int cpu_post_load(void *opaque, int version_id)
> > if (cpu->compat_pvr) {
> > Error *local_err = NULL;
> >
> > - ppc_set_compat(cpu, cpu->compat_pvr, &local_err);
> > + ppc_set_compat(cpu, cpu->compat_pvr, false, &local_err);
> > if (local_err) {
> > error_report_err(local_err);
> > error_free(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;
> > > +}
> > > +
> > > +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
> > > }
> > > };
> >
>
>
>
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