tools/libxc: Reduce feature handling complexity in xc_cpuid_apply_policy()

xc_cpuid_apply_policy() is gaining extra parameters to untangle CPUID
complexity in Xen.  While an improvement in general, it does have the
unfortunate side effect of duplicating some settings across multiple
parameters.

Rearrange the logic to only consider 'pae' if no explicit featureset is
provided.  This reduces the complexity for callers who have already provided a
pae setting in the featureset.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <pdurrant@amzn.com>
Acked-by: Wei Liu <wl@xen.org>

tools/libxc/include/xenctrl.h

@@ -1798,6 +1798,13 @@ int xc_cpuid_set(xc_interface *xch,
                  const unsigned int *input,
                  const char **config,
                  char **config_transformed);
+
+/*
+ * Make adjustments to the CPUID settings for a domain.
+ *
+ * Either pass a full new @featureset (and @nr_features), or adjust individual
+ * features (@pae).
+ */
 int xc_cpuid_apply_policy(xc_interface *xch,
                           uint32_t domid,
                           const uint32_t *featureset,

tools/libxc/xc_cpuid_x86.c

@@ -532,6 +532,11 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid,
 
         cpuid_featureset_to_policy(feat, p);
     }
+    else
+    {
+        if ( di.hvm )
+            p->basic.pae = pae;
+    }
 
     if ( !di.hvm )
     {
@@ -615,8 +620,6 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid,
             break;
         }
 
-        p->basic.pae = pae;
-
         /*
          * These settings are necessary to cause earlier HVM_PARAM_NESTEDHVM /
          * XEN_DOMCTL_disable_migrate settings to be reflected correctly in