x86/cpuidle: clean up statistics reporting to user mode

First of all, when no ACPI Cx data was reported, make sure the usage
count passed back to user mode is not random.

Besides that, fold a lot of redundant code.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -1100,36 +1100,23 @@ int pmstat_get_cx_stat(uint32_t cpuid, s
     }
 
     stat->last = power->last_state ? power->last_state->idx : 0;
-    stat->nr = power->count;
     stat->idle_time = get_cpu_idle_time(cpuid);
 
     /* mimic the stat when detail info hasn't been registered by dom0 */
     if ( pm_idle_save == NULL )
     {
-        /* C1 */
-        usage[1] = 1;
-        res[1] = stat->idle_time;
-
-        /* C0 */
-        res[0] = NOW() - res[1];
-
-        if ( copy_to_guest_offset(stat->triggers, 0, &usage[0], 2) ||
-            copy_to_guest_offset(stat->residencies, 0, &res[0], 2) )
-            return -EFAULT;
-
-        stat->pc2 = 0;
-        stat->pc3 = 0;
-        stat->pc6 = 0;
-        stat->pc7 = 0;
-        stat->cc3 = 0;
-        stat->cc6 = 0;
-        stat->cc7 = 0;
-        return 0;
-    }
+        stat->nr = 2;
+
+        usage[1] = idle_usage = 1;
+        res[1] = idle_res = stat->idle_time;
 
-    for ( i = power->count - 1; i >= 0; i-- )
+        memset(&hw_res, 0, sizeof(hw_res));
+    }
+    else
     {
-        if ( i != 0 )
+        stat->nr = power->count;
+
+        for ( i = 1; i < power->count; i++ )
         {
             spin_lock_irq(&power->stat_lock);
             usage[i] = power->states[i].usage;
@@ -1139,18 +1126,16 @@ int pmstat_get_cx_stat(uint32_t cpuid, s
             idle_usage += usage[i];
             idle_res += res[i];
         }
-        else
-        {
-            usage[i] = idle_usage;
-            res[i] = NOW() - idle_res;
-        }
+
+        get_hw_residencies(cpuid, &hw_res);
     }
 
-    if ( copy_to_guest_offset(stat->triggers, 0, &usage[0], power->count) ||
-        copy_to_guest_offset(stat->residencies, 0, &res[0], power->count) )
-        return -EFAULT;
+    usage[0] = idle_usage;
+    res[0] = NOW() - idle_res;
 
-    get_hw_residencies(cpuid, &hw_res);
+    if ( copy_to_guest(stat->triggers, usage, stat->nr) ||
+         copy_to_guest(stat->residencies, res, stat->nr) )
+        return -EFAULT;
 
     stat->pc2 = hw_res.pc2;
     stat->pc3 = hw_res.pc3;