qemu: Fix CPU model broken by older libvirt

When libvirt older than 3.9.0 reconnected to a running domain started by
old libvirt it could have messed up the expansion of host-model by
adding features QEMU does not support (such as cmt). Thus whenever we
reconnect to a running domain, revert to an active snapshot, or restore
a saved domain we need to check the guest CPU model and remove the
CPU features unknown to QEMU. We can do this because we know the domain
was successfully started, which means the CPU did not contain the
features when libvirt started the domain.

https://bugzilla.redhat.com/show_bug.cgi?id=1495171

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

src/qemu/qemu_domain.c

@@ -10027,6 +10027,82 @@ qemuDomainUpdateCPU(virDomainObjPtr vm,
     return 0;
 }
 
+
+/**
+ * qemuDomainFixupCPUS:
+ * @vm: domain object
+ * @origCPU: original CPU used when the domain was started
+ *
+ * Libvirt older than 3.9.0 could have messed up the expansion of host-model
+ * CPU when reconnecting to a running domain by adding features QEMU does not
+ * support (such as cmt). This API fixes both the actual CPU provided by QEMU
+ * (stored in the domain object) and the @origCPU used when starting the
+ * domain.
+ *
+ * This is safe even if the original CPU definition used mode='custom' (rather
+ * than host-model) since we know QEMU was able to start the domain and thus
+ * the CPU definitions do not contain any features unknown to QEMU.
+ *
+ * This function can only be used on an active domain or when restoring a
+ * domain which was running.
+ *
+ * Returns 0 on success, -1 on error.
+ */
+int
+qemuDomainFixupCPUs(virDomainObjPtr vm,
+                    virCPUDefPtr *origCPU)
+{
+    virCPUDefPtr fixedCPU = NULL;
+    virCPUDefPtr fixedOrig = NULL;
+    virArch arch = vm->def->os.arch;
+    int ret = 0;
+
+    if (!ARCH_IS_X86(arch))
+        return 0;
+
+    if (!vm->def->cpu ||
+        vm->def->cpu->mode != VIR_CPU_MODE_CUSTOM ||
+        !vm->def->cpu->model)
+        return 0;
+
+    /* Missing origCPU means QEMU created exactly the same virtual CPU which
+     * we asked for or libvirt was too old to mess up the translation from
+     * host-model.
+     */
+    if (!*origCPU)
+        return 0;
+
+    if (virCPUDefFindFeature(vm->def->cpu, "cmt") &&
+        (!(fixedCPU = virCPUDefCopyWithoutModel(vm->def->cpu)) ||
+         virCPUDefCopyModelFilter(fixedCPU, vm->def->cpu, false,
+                                  virQEMUCapsCPUFilterFeatures, &arch) < 0))
+        goto cleanup;
+
+    if (virCPUDefFindFeature(*origCPU, "cmt") &&
+        (!(fixedOrig = virCPUDefCopyWithoutModel(*origCPU)) ||
+         virCPUDefCopyModelFilter(fixedOrig, *origCPU, false,
+                                  virQEMUCapsCPUFilterFeatures, &arch) < 0))
+        goto cleanup;
+
+    if (fixedCPU) {
+        virCPUDefFree(vm->def->cpu);
+        VIR_STEAL_PTR(vm->def->cpu, fixedCPU);
+    }
+
+    if (fixedOrig) {
+        virCPUDefFree(*origCPU);
+        VIR_STEAL_PTR(*origCPU, fixedOrig);
+    }
+
+    ret = 0;
+
+ cleanup:
+    virCPUDefFree(fixedCPU);
+    virCPUDefFree(fixedOrig);
+    return ret;
+}
+
+
 char *
 qemuDomainGetMachineName(virDomainObjPtr vm)
 {

src/qemu/qemu_domain.h

@@ -971,6 +971,10 @@ qemuDomainUpdateCPU(virDomainObjPtr vm,
                     virCPUDefPtr cpu,
                     virCPUDefPtr *origCPU);
 
+int
+qemuDomainFixupCPUs(virDomainObjPtr vm,
+                    virCPUDefPtr *origCPU);
+
 char *
 qemuDomainGetMachineName(virDomainObjPtr vm);
 

src/qemu/qemu_driver.c

@@ -6551,6 +6551,13 @@ qemuDomainSaveImageStartVM(virConnectPtr conn,
         }
     }
 
+    /* No cookie means libvirt which saved the domain was too old to mess up
+     * the CPU definitions.
+     */
+    if (cookie &&
+        qemuDomainFixupCPUs(vm, &cookie->cpu) < 0)
+        goto cleanup;
+
     if (qemuProcessStart(conn, driver, vm, cookie ? cookie->cpu : NULL,
                          asyncJob, "stdio", *fd, path, NULL,
                          VIR_NETDEV_VPORT_PROFILE_OP_RESTORE,
@@ -15756,6 +15763,13 @@ qemuDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,
             if (config)
                 virDomainObjAssignDef(vm, config, false, NULL);
 
+            /* No cookie means libvirt which saved the domain was too old to
+             * mess up the CPU definitions.
+             */
+            if (cookie &&
+                qemuDomainFixupCPUs(vm, &cookie->cpu) < 0)
+                goto cleanup;
+
             rc = qemuProcessStart(snapshot->domain->conn, driver, vm,
                                   cookie ? cookie->cpu : NULL,
                                   QEMU_ASYNC_JOB_START, NULL, -1, NULL, snap,

src/qemu/qemu_process.c

@@ -6881,6 +6881,7 @@ qemuProcessRefreshCPU(virQEMUDriverPtr driver,
                       virDomainObjPtr vm)
 {
     virCapsPtr caps = virQEMUDriverGetCapabilities(driver, false);
+    qemuDomainObjPrivatePtr priv = vm->privateData;
     virCPUDefPtr host = NULL;
     virCPUDefPtr cpu = NULL;
     int ret = -1;
@@ -6915,6 +6916,14 @@ qemuProcessRefreshCPU(virQEMUDriverPtr driver,
 
         if (qemuProcessUpdateCPU(driver, vm, QEMU_ASYNC_JOB_NONE) < 0)
             goto cleanup;
+    } else if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_QUERY_CPU_MODEL_EXPANSION)) {
+        /* We only try to fix CPUs when the libvirt/QEMU combo used to start
+         * the domain did not know about query-cpu-model-expansion in which
+         * case the host-model is known to not contain features which QEMU
+         * doesn't know about.
+         */
+        if (qemuDomainFixupCPUs(vm, &priv->origCPU) < 0)
+            goto cleanup;
     }
 
     ret = 0;