PPC/KVM: early validation of vcpu id

The KVM API restricts vcpu ids to be < KVM_CAP_MAX_VCPUS. On PowerPC
targets, depending on the number of threads per core in the host and
in the guest, some topologies do generate higher vcpu ids actually.
When this happens, QEMU bails out with the following error:

kvm_init_vcpu failed: Invalid argument

The KVM_CREATE_VCPU ioctl has several EINVAL return paths, so it is
not possible to fully disambiguate.

This patch adds a check in the code that computes vcpu ids, so that
we can detect the error earlier, and print a friendlier message instead
of calling KVM_CREATE_VCPU with an obviously bogus vcpu id.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

include/sysemu/kvm.h

@@ -345,6 +345,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s);
 
 int kvm_arch_init_vcpu(CPUState *cpu);
 
+bool kvm_vcpu_id_is_valid(int vcpu_id);
+
 /* Returns VCPU ID to be used on KVM_CREATE_VCPU ioctl() */
 unsigned long kvm_arch_vcpu_id(CPUState *cpu);
 

kvm-all.c

@@ -1459,6 +1459,12 @@ static int kvm_max_vcpus(KVMState *s)
     return (ret) ? ret : kvm_recommended_vcpus(s);
 }
 
+bool kvm_vcpu_id_is_valid(int vcpu_id)
+{
+    KVMState *s = KVM_STATE(current_machine->accelerator);
+    return vcpu_id >= 0 && vcpu_id < kvm_max_vcpus(s);
+}
+
 static int kvm_init(MachineState *ms)
 {
     MachineClass *mc = MACHINE_GET_CLASS(ms);

target-ppc/translate_init.c

@@ -9231,6 +9231,14 @@ static void ppc_cpu_realizefn(DeviceState *dev, Error **errp)
 #if !defined(CONFIG_USER_ONLY)
     cpu->cpu_dt_id = (cs->cpu_index / smp_threads) * max_smt
         + (cs->cpu_index % smp_threads);
+
+    if (kvm_enabled() && !kvm_vcpu_id_is_valid(cpu->cpu_dt_id)) {
+        error_setg(errp, "Can't create CPU with id %d in KVM", cpu->cpu_dt_id);
+        error_append_hint(errp, "Adjust the number of cpus to %d "
+                          "or try to raise the number of threads per core\n",
+                          cpu->cpu_dt_id * smp_threads / max_smt);
+        return;
+    }
 #endif
 
     if (tcg_enabled()) {