x86: fix guest CPUID handling

The way XEN_DOMCTL_set_cpuid got handled so far allowed for surprises
to the caller. With this set of operations
- set leaf A (using array index 0)
- set leaf B (using array index 1)
- clear leaf A (clearing array index 0)
- set leaf B (using array index 0)
- clear leaf B (clearing array index 0)
the entry for leaf B at array index 1 would still be in place, while
the caller would expect it to be cleared.

While looking at the use sites of d->arch.cpuid[] I also noticed that
the allocation of the array needlessly uses the zeroing form - the
relevant fields of the array elements get set in a loop immediately
following the allocation.

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

--- 2014-04-16.orig/xen/arch/x86/domain.c	2014-02-05 15:21:00.000000000 +0100
+++ 2014-04-16/xen/arch/x86/domain.c	2014-04-30 13:53:18.000000000 +0200
@@ -549,7 +549,7 @@ int arch_domain_create(struct domain *d,
 
     if ( !is_idle_domain(d) )
     {
-        d->arch.cpuids = xzalloc_array(cpuid_input_t, MAX_CPUID_INPUT);
+        d->arch.cpuids = xmalloc_array(cpuid_input_t, MAX_CPUID_INPUT);
         rc = -ENOMEM;
         if ( d->arch.cpuids == NULL )
             goto fail;
--- 2014-04-16.orig/xen/arch/x86/domctl.c	2014-02-05 15:21:01.000000000 +0100
+++ 2014-04-16/xen/arch/x86/domctl.c	2014-04-30 13:43:30.000000000 +0200
@@ -1006,14 +1006,18 @@ long arch_do_domctl(
     case XEN_DOMCTL_set_cpuid:
     {
         xen_domctl_cpuid_t *ctl = &domctl->u.cpuid;
-        cpuid_input_t *cpuid = NULL; 
+        cpuid_input_t *cpuid, *unused = NULL;
 
         for ( i = 0; i < MAX_CPUID_INPUT; i++ )
         {
             cpuid = &d->arch.cpuids[i];
 
             if ( cpuid->input[0] == XEN_CPUID_INPUT_UNUSED )
-                break;
+            {
+                if ( !unused )
+                    unused = cpuid;
+                continue;
+            }
 
             if ( (cpuid->input[0] == ctl->input[0]) &&
                  ((cpuid->input[1] == XEN_CPUID_INPUT_UNUSED) ||
@@ -1021,15 +1025,12 @@ long arch_do_domctl(
                 break;
         }
         
-        if ( i == MAX_CPUID_INPUT )
-        {
-            ret = -ENOENT;
-        }
+        if ( i < MAX_CPUID_INPUT )
+            *cpuid = *ctl;
+        else if ( unused )
+            *unused = *ctl;
         else
-        {
-            memcpy(cpuid, ctl, sizeof(cpuid_input_t));
-            ret = 0;
-        }
+            ret = -ENOENT;
     }
     break;