spapr/xics: Configure number of servers in KVM

The XICS-on-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
and pre-POWER9 XICS KVM devices.

The XICS-on-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: <157478678846.67101.9660531022460517710.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/xics_kvm.c

@@ -165,8 +165,15 @@ void icp_kvm_realize(DeviceState *dev, Error **errp)
 
     ret = kvm_vcpu_enable_cap(cs, KVM_CAP_IRQ_XICS, 0, kernel_xics_fd, vcpu_id);
     if (ret < 0) {
-        error_setg(errp, "Unable to connect CPU%ld to kernel XICS: %s", vcpu_id,
-                   strerror(errno));
+        Error *local_err = NULL;
+
+        error_setg(&local_err, "Unable to connect CPU%ld to kernel XICS: %s",
+                   vcpu_id, strerror(errno));
+        if (errno == ENOSPC) {
+            error_append_hint(&local_err, "Try -smp maxcpus=N with N < %u\n",
+                              MACHINE(qdev_get_machine())->smp.max_cpus);
+        }
+        error_propagate(errp, local_err);
         return;
     }
     enabled_icp = g_malloc(sizeof(*enabled_icp));
@@ -399,6 +406,16 @@ int xics_kvm_connect(SpaprInterruptController *intc, uint32_t nr_servers,
         goto fail;
     }
 
+    /* Tell KVM about the # of VCPUs we may have (POWER9 and newer only) */
+    if (kvm_device_check_attr(rc, KVM_DEV_XICS_GRP_CTRL,
+                              KVM_DEV_XICS_NR_SERVERS)) {
+        if (kvm_device_access(rc, KVM_DEV_XICS_GRP_CTRL,
+                              KVM_DEV_XICS_NR_SERVERS, &nr_servers, true,
+                              &local_err)) {
+            goto fail;
+        }
+    }
+
     kernel_xics_fd = rc;
     kvm_kernel_irqchip = true;
     kvm_msi_via_irqfd_allowed = true;