On a POWER9 host, if a guest runs in pre POWER9 compat mode, it necessarily
uses the hash MMU mode. In this case, we shouldn't advertise radix GTSE in
the ibm,arch-vec-5-platform-support DT property as the current code does.
The first reason is that it doesn't make sense, and the second one is that
causes the CAS-negotiated options subsection to be migrated. This breaks
backward migration to QEMU 2.7 and older versions on POWER8 hosts:

qemu-system-ppc64: error while loading state for instance 0x0 of device
 'spapr'
qemu-system-ppc64: load of migration failed: No such file or directory

This patch hence initialize CPUs a bit earlier so that we can check the
requested compat mode, and don't set OV5_MMU_RADIX_GTSE for power8 and
older.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 hw/ppc/spapr.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index eb9d087d1a60..154bf9d548c4 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2517,6 +2517,7 @@ static void spapr_machine_init(MachineState *machine)
     long load_limit, fw_size;
     char *filename;
     Error *resize_hpt_err = NULL;
+    PowerPCCPU *first_ppc_cpu;
 
     msi_nonbroken = true;
 
@@ -2609,11 +2610,6 @@ static void spapr_machine_init(MachineState *machine)
     }
 
     spapr_ovec_set(spapr->ov5, OV5_FORM1_AFFINITY);
-    if (!kvm_enabled() || kvmppc_has_cap_mmu_radix()) {
-        /* KVM and TCG always allow GTSE with radix... */
-        spapr_ovec_set(spapr->ov5, OV5_MMU_RADIX_GTSE);
-    }
-    /* ... but not with hash (currently). */
 
     /* advertise support for dedicated HP event source to guests */
     if (spapr->use_hotplug_event_source) {
@@ -2631,6 +2627,15 @@ static void spapr_machine_init(MachineState *machine)
     /* init CPUs */
     spapr_init_cpus(spapr);
 
+    first_ppc_cpu = POWERPC_CPU(first_cpu);
+    if ((!kvm_enabled() || kvmppc_has_cap_mmu_radix()) &&
+        ppc_check_compat(first_ppc_cpu, CPU_POWERPC_LOGICAL_3_00, 0,
+                         spapr->max_compat_pvr)) {
+        /* KVM and TCG always allow GTSE with radix... */
+        spapr_ovec_set(spapr->ov5, OV5_MMU_RADIX_GTSE);
+    }
+    /* ... but not with hash (currently). */
+
     if (kvm_enabled()) {
         /* Enable H_LOGICAL_CI_* so SLOF can talk to in-kernel devices */
         kvmppc_enable_logical_ci_hcalls();