kvm: better advice for failed s390x startup

If KVM_CREATE failed on s390x, we print a hint to enable the switch_amode
kernel parameter. This only applies to old kernels, and only if the
error was -EINVAL. Moreover, with new kernels, the most likely reason
for -EINVAL is that pgstes were not enabled.

Let's update the error message to give a better hint on where things
may need fixing.
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>

kvm-all.c

@@ -1544,8 +1544,17 @@ static int kvm_init(MachineState *ms)
                 strerror(-ret));
 
 #ifdef TARGET_S390X
-        fprintf(stderr, "Please add the 'switch_amode' kernel parameter to "
-                        "your host kernel command line\n");
+        if (ret == -EINVAL) {
+            fprintf(stderr,
+                    "Host kernel setup problem detected. Please verify:\n");
+            fprintf(stderr, "- for kernels supporting the switch_amode or"
+                    " user_mode parameters, whether\n");
+            fprintf(stderr,
+                    "  user space is running in primary address space\n");
+            fprintf(stderr,
+                    "- for kernels supporting the vm.allocate_pgste sysctl, "
+                    "whether it is enabled\n");
+        }
 #endif
         goto err;
     }