s390x: Limit s390-ccw machines to 248 CPUs

The sclp scp read info call fills in a buffer with information about the
system. With more than 248 CPUs we overflow the 4k buffer of the SCCB,
leading to random data corruption. Basically ALL guest operating systems
call scp read info, so let's limit the machines to 248 CPUs to make it
obvious that >=249 does not work.

As KVM also limits itself to 248 and TCG on s390 does not support
SMP, this should cause no regression for any user as no VMs with more
than 248 VCPUs were ever possible.

Signed-off-by: Christian Borntraeger <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Boris Fiuczynski <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 95ff5e35ae97c856dbc031ccab74fe5e5b446caf..e257ca5ab0cbd017d43c20456ea02b6621ea3918 100644 (file)
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -204,7 +204,7 @@ static void ccw_machine_class_init(ObjectClass *oc, void *data)
     mc->no_parallel = 1;
     mc->no_sdcard = 1;
     mc->use_sclp = 1;
-    mc->max_cpus = 255;
+    mc->max_cpus = 248;
     mc->get_hotplug_handler = s390_get_hotplug_handler;
     hc->plug = s390_machine_device_plug;
     nc->nmi_monitor_handler = s390_nmi;