qemu: domain: Drop added features from migratable CPU

Features marked with added='yes' in CPU model definitions have to be
removed before migration, otherwise older libvirt would complain about
unknown CPU features. We only do this for features that were enabled for
a given CPU model even with older libvirt, which just ignored the
features. And only for features we added ourselves when updating CPU
definition during domain startup, that is we do not remove features
which were explicitly mentioned by a user.

That said, this is not the safest thing we could do, but it's
effectively the same thing we did before the affected features were
added: we ignored them completely on both sides of migration.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/qemu/qemu_domain.c

@@ -66,6 +66,7 @@
 #include "backup_conf.h"
 #include "virutil.h"
 #include "virsecureerase.h"
+#include "cpu/cpu_x86.h"
 
 #include <sys/time.h>
 #include <fcntl.h>
@@ -6643,10 +6644,36 @@ qemuDomainDefCopy(virQEMUDriver *driver,
 }
 
 
+typedef struct {
+    const char * const *added;
+    GStrv keep;
+} qemuDomainDropAddedCPUFeaturesData;
+
+
+static bool
+qemuDomainDropAddedCPUFeatures(const char *name,
+                               virCPUFeaturePolicy policy G_GNUC_UNUSED,
+                               void *opaque)
+{
+    qemuDomainDropAddedCPUFeaturesData *data = opaque;
+
+    if (!g_strv_contains(data->added, name))
+        return true;
+
+    if (data->keep && g_strv_contains((const char **) data->keep, name))
+        return true;
+
+    return false;
+}
+
+
 int
 qemuDomainMakeCPUMigratable(virArch arch,
-                            virCPUDef *cpu)
+                            virCPUDef *cpu,
+                            virCPUDef *origCPU)
 {
+    qemuDomainDropAddedCPUFeaturesData data = { 0 };
+
     if (cpu->mode != VIR_CPU_MODE_CUSTOM ||
         !cpu->model ||
         !ARCH_IS_X86(arch))
@@ -6664,6 +6691,25 @@ qemuDomainMakeCPUMigratable(virArch arch,
             return -1;
     }
 
+    if (virCPUx86GetAddedFeatures(cpu->model, &data.added) < 0)
+        return -1;
+
+    /* Drop features marked as added in a cpu model, but only
+     * when they are not mentioned in origCPU, i.e., when they were not
+     * explicitly mentioned by the user.
+     */
+    if (data.added) {
+        g_auto(GStrv) keep = NULL;
+
+        if (origCPU) {
+            keep = virCPUDefListExplicitFeatures(origCPU);
+            data.keep = keep;
+        }
+
+        if (virCPUDefFilterFeatures(cpu, qemuDomainDropAddedCPUFeatures, &data) < 0)
+            return -1;
+    }
+
     return 0;
 }
 
@@ -6843,7 +6889,7 @@ qemuDomainDefFormatBufInternal(virQEMUDriver *driver,
         }
 
         if (def->cpu &&
-            qemuDomainMakeCPUMigratable(def->os.arch, def->cpu) < 0)
+            qemuDomainMakeCPUMigratable(def->os.arch, def->cpu, origCPU) < 0)
             return -1;
 
         /* Old libvirt doesn't understand <audio> elements so

src/qemu/qemu_domain.h

@@ -1049,7 +1049,8 @@ qemuDomainSupportsCheckpointsBlockjobs(virDomainObj *vm)
 
 int
 qemuDomainMakeCPUMigratable(virArch arch,
-                            virCPUDef *cpu);
+                            virCPUDef *cpu,
+                            virCPUDef *origCPU);
 
 int
 qemuDomainInitializePflashStorageSource(virDomainObj *vm,

src/qemu/qemu_migration_cookie.c

@@ -540,12 +540,15 @@ static int
 qemuMigrationCookieAddCPU(qemuMigrationCookie *mig,
                           virDomainObj *vm)
 {
+    qemuDomainObjPrivate *priv = vm->privateData;
+
     if (mig->cpu)
         return 0;
 
     mig->cpu = virCPUDefCopy(vm->def->cpu);
 
-    if (qemuDomainMakeCPUMigratable(vm->def->os.arch, mig->cpu) < 0)
+    if (qemuDomainMakeCPUMigratable(vm->def->os.arch, mig->cpu,
+                                    priv->origCPU) < 0)
         return -1;
 
     mig->flags |= QEMU_MIGRATION_COOKIE_CPU;