cpu: Let explicit features override model features

Until now CPU features inherited from a specified CPU model could only
be overridden with 'disable' policy. With this patch, any explicitly
specified feature always overrides the same feature inherited from a CPU
model regardless on the specified policy.

The CPU in x86-exact-force-Haswell.xml would previously be incompatible
with x86-host-SandyBridge.xml CPU even though x86-host-SandyBridge.xml
provides all features required by x86-exact-force-Haswell.xml.

src/cpu/cpu_x86.c

@@ -1223,6 +1223,11 @@ x86Compute(virCPUDefPtr host,
         goto out;
     }
 
+    /* first remove features that were inherited from the CPU model and were
+     * explicitly forced, disabled, or made optional
+     */
+    x86DataSubtract(cpu_require->data, cpu_force->data);
+    x86DataSubtract(cpu_require->data, cpu_optional->data);
     x86DataSubtract(cpu_require->data, cpu_disable->data);
     result = x86ModelCompare(host_model, cpu_require);
     if (result == SUBSET || result == UNRELATED) {

tests/cputest.c

@@ -581,6 +581,7 @@ mymain(void)
     DO_TEST_COMPARE("x86", "host", "pentium3-amd", VIR_CPU_COMPARE_INCOMPATIBLE);
     DO_TEST_COMPARE("x86", "host-amd", "pentium3-amd", VIR_CPU_COMPARE_SUPERSET);
     DO_TEST_COMPARE("x86", "host-worse", "nehalem-force", VIR_CPU_COMPARE_IDENTICAL);
+    DO_TEST_COMPARE("x86", "host-SandyBridge", "exact-force-Haswell", VIR_CPU_COMPARE_IDENTICAL);
 
     /* guest updates for migration
      * automatically compares host CPU with the result */

tests/cputestdata/x86-exact-force-Haswell.xml

+<cpu mode='custom' match='exact'>
+  <model fallback='allow'>Haswell</model>
+  <feature policy='force' name='pcid'/>
+  <feature policy='force' name='rtm'/>
+  <feature policy='force' name='invpcid'/>
+  <feature policy='force' name='erms'/>
+  <feature policy='force' name='bmi2'/>
+  <feature policy='optional' name='smep'/>
+  <feature policy='optional' name='avx2'/>
+  <feature policy='optional' name='hle'/>
+  <feature policy='force' name='bmi1'/>
+  <feature policy='force' name='fsgsbase'/>
+  <feature policy='disable' name='movbe'/>
+  <feature policy='disable' name='fma'/>
+</cpu>

tests/cputestdata/x86-host-SandyBridge.xml

+<cpu>
+  <arch>x86_64</arch>
+  <model>SandyBridge</model>
+  <vendor>Intel</vendor>
+  <topology sockets='1' cores='2' threads='2'/>
+</cpu>