spapr/xive: Configure number of servers in KVM

The XIVE KVM devices now has an attribute to configure the number of
interrupt servers. This allows to greatly optimize the usage of the VP
space in the XIVE HW, and thus to start a lot more VMs.

Only set this attribute if available in order to support older POWER9
KVM.

The XIVE KVM device now reports the exhaustion of VPs upon the
connection of the first VCPU. Check that in order to have a chance
to provide a hint to the user.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <157478679392.67101.7843580591407950866.stgit@bahia.tlslab.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/intc/spapr_xive_kvm.c

@@ -152,7 +152,8 @@ void kvmppc_xive_cpu_synchronize_state(XiveTCTX *tctx, Error **errp)
 
 void kvmppc_xive_cpu_connect(XiveTCTX *tctx, Error **errp)
 {
-    SpaprXive *xive = SPAPR_MACHINE(qdev_get_machine())->xive;
+    MachineState *ms = MACHINE(qdev_get_machine());
+    SpaprXive *xive = SPAPR_MACHINE(ms)->xive;
     unsigned long vcpu_id;
     int ret;
 
@@ -171,8 +172,16 @@ void kvmppc_xive_cpu_connect(XiveTCTX *tctx, Error **errp)
     ret = kvm_vcpu_enable_cap(tctx->cs, KVM_CAP_PPC_IRQ_XIVE, 0, xive->fd,
                               vcpu_id, 0);
     if (ret < 0) {
-        error_setg(errp, "XIVE: unable to connect CPU%ld to KVM device: %s",
+        Error *local_err = NULL;
+
+        error_setg(&local_err,
+                   "XIVE: unable to connect CPU%ld to KVM device: %s",
                    vcpu_id, strerror(errno));
+        if (errno == ENOSPC) {
+            error_append_hint(&local_err, "Try -smp maxcpus=N with N < %u\n",
+                              ms->smp.max_cpus);
+        }
+        error_propagate(errp, local_err);
         return;
     }
 
@@ -758,6 +767,16 @@ int kvmppc_xive_connect(SpaprInterruptController *intc, uint32_t nr_servers,
         return -1;
     }
 
+    /* Tell KVM about the # of VCPUs we may have */
+    if (kvm_device_check_attr(xive->fd, KVM_DEV_XIVE_GRP_CTRL,
+                              KVM_DEV_XIVE_NR_SERVERS)) {
+        if (kvm_device_access(xive->fd, KVM_DEV_XIVE_GRP_CTRL,
+                              KVM_DEV_XIVE_NR_SERVERS, &nr_servers, true,
+                              &local_err)) {
+            goto fail;
+        }
+    }
+
     /*
      * 1. Source ESB pages - KVM mapping
      */