cpu: Try to use source CPU model in virConnectBaselineCPU

https://bugzilla.redhat.com/show_bug.cgi?id=1049391

When all source CPU XMLs contain just a single CPU model (with a
possibly varying set of additional feature elements),
virConnectBaselineCPU will try to use this CPU model in the computed
guest CPU. Thus, when used on just a single CPU (useful with
VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES), the result will not use a
different CPU model.

If the computed CPU uses the source model, set fallback mode to 'forbid'
to make sure the guest CPU will always be as close as possible to the
source CPUs.

src/cpu/cpu_x86.c

@@ -1851,6 +1851,8 @@ x86Baseline(virCPUDefPtr *cpus,
     const struct x86_vendor *vendor = NULL;
     struct x86_model *model = NULL;
     bool outputVendor = true;
+    const char *modelName;
+    bool matchingNames = true;
 
     if (!(map = virCPUx86GetMap()))
         goto error;
@@ -1873,9 +1875,19 @@ x86Baseline(virCPUDefPtr *cpus,
         goto error;
     }
 
+    modelName = cpus[0]->model;
     for (i = 1; i < ncpus; i++) {
         const char *vn = NULL;
 
+        if (matchingNames && cpus[i]->model) {
+            if (!modelName) {
+                modelName = cpus[i]->model;
+            } else if (STRNEQ(modelName, cpus[i]->model)) {
+                modelName = NULL;
+                matchingNames = false;
+            }
+        }
+
         if (!(model = x86ModelFromCPU(cpus[i], map, VIR_CPU_FEATURE_REQUIRE)))
             goto error;
 
@@ -1923,9 +1935,12 @@ x86Baseline(virCPUDefPtr *cpus,
     if (vendor && virCPUx86DataAddCPUID(base_model->data, &vendor->cpuid) < 0)
         goto error;
 
-    if (x86Decode(cpu, base_model->data, models, nmodels, NULL, flags) < 0)
+    if (x86Decode(cpu, base_model->data, models, nmodels, modelName, flags) < 0)
         goto error;
 
+    if (STREQ_NULLABLE(cpu->model, modelName))
+        cpu->fallback = VIR_CPU_FALLBACK_FORBID;
+
     if (!outputVendor)
         VIR_FREE(cpu->vendor);
 

tests/cputestdata/x86-baseline-3-expanded.xml

 <cpu mode='custom' match='exact'>
-  <model fallback='allow'>Westmere</model>
+  <model fallback='forbid'>Westmere</model>
   <feature policy='require' name='lahf_lm'/>
   <feature policy='require' name='lm'/>
   <feature policy='require' name='nx'/>