1 files changed, 15 insertions, 0 deletions

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 5fac6919b957..0503c0c493a9 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -178,6 +178,7 @@ static void __init xen_fill_possible_map(void)
 static void __init xen_filter_cpu_maps(void)
 {
         int i, rc;
+        unsigned int subtract = 0;
 
         if (!xen_initial_domain())
                 return;
@@ -192,8 +193,22 @@ static void __init xen_filter_cpu_maps(void)
                 } else {
                         set_cpu_possible(i, false);
                         set_cpu_present(i, false);
+                        subtract++;
                 }
         }
+#ifdef CONFIG_HOTPLUG_CPU
+        /* This is akin to using 'nr_cpus' on the Linux command line.
+         * Which is OK as when we use 'dom0_max_vcpus=X' we can only
+         * have up to X, while nr_cpu_ids is greater than X. This
+         * normally is not a problem, except when CPU hotplugging
+         * is involved and then there might be more than X CPUs
+         * in the guest - which will not work as there is no
+         * hypercall to expand the max number of VCPUs an already
+         * running guest has. So cap it up to X. */
+        if (subtract)
+                nr_cpu_ids = nr_cpu_ids - subtract;
+#endif
+
 }
 
 static void __init xen_smp_prepare_boot_cpu(void)