xen/sched: reject switching smt on/off with core scheduling active

When core or socket scheduling are active enabling or disabling smt is
not possible as that would require a major host reconfiguration.

Add a bool sched_disable_smt_switching which will be set for core or
socket scheduling.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>

xen/arch/x86/sysctl.c

@@ -209,6 +209,11 @@ long arch_do_sysctl(
                 ret = -EOPNOTSUPP;
                 break;
             }
+            if ( sched_disable_smt_switching )
+            {
+                ret = -EBUSY;
+                break;
+            }
             plug = op == XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE;
             fn = smt_up_down_helper;
             hcpu = _p(plug);

xen/common/schedule.c

@@ -63,6 +63,7 @@ integer_param("sched_ratelimit_us", sched_ratelimit_us);
 
 /* Number of vcpus per struct sched_unit. */
 static unsigned int __read_mostly sched_granularity = 1;
+bool __read_mostly sched_disable_smt_switching;
 const cpumask_t *sched_res_mask = &cpumask_all;
 
 /* Common lock for free cpus. */

xen/include/xen/sched.h

@@ -1037,6 +1037,7 @@ static inline bool is_iommu_enabled(const struct domain *d)
 }
 
 extern bool sched_smt_power_savings;
+extern bool sched_disable_smt_switching;
 
 extern enum cpufreq_controller {
     FREQCTL_none, FREQCTL_dom0_kernel, FREQCTL_xen