qemu: add runtime config option for nbdkit

Currently when we build with nbdkit support, libvirt will always try to
use nbdkit to access remote disk sources when it is available. But
without an up-to-date selinux policy allowing this, it will fail.
because the required selinux policies are not yet widely available, we
have disabled nbdkit support on rpm builds for all distributions before
Fedora 40.

Unfortunately, this makes it more difficult to test nbdkit support.
After someone updates to the necessary selinux policies, they would also
need to rebuild libvirt to enable nbdkit support. By introducing a
configure option (nbdkit_config_default), we can build packages with
nbdkit support but have it disabled by default.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Suggested-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>

libvirt.spec.in

@@ -96,6 +96,7 @@
 %define with_sanlock          0
 %define with_numad            0
 %define with_nbdkit           0
+%define with_nbdkit_config_default 0
 %define with_firewalld_zone   0
 %define with_netcf            0
 %define with_libssh2          0
@@ -174,15 +175,17 @@
     %endif
 %endif
 
-# We should only enable nbdkit support if the OS ships a SELinux policy that
-# allows libvirt to launch it. Right now that's not the case anywhere, but
-# things should be fine by the time Fedora 40 is released.
+# We want to build with nbdkit support, but should only enable nbdkit by
+# default if the OS ships a SELinux policy that allows libvirt to launch it.
+# Right now that's not the case anywhere, but things should be fine by the time
+# Fedora 40 is released.
 #
 # TODO: add RHEL 9 once a minor release that contains the necessary SELinux
 #       bits exists (we only support the most recent minor release)
 %if %{with_qemu}
+    %define with_nbdkit 0%{!?_without_nbdkit:1}
     %if 0%{?fedora} >= 40
-        %define with_nbdkit 0%{!?_without_nbdkit:1}
+        %define with_nbdkit_config_default 0%{!?_without_nbdkit_config_default:1}
     %endif
 %endif
 
@@ -1207,6 +1210,12 @@ exit 1
     %define arg_nbdkit -Dnbdkit=disabled
 %endif
 
+%if %{with_nbdkit_config_default}
+    %define arg_nbdkit_config_default -Dnbdkit_config_default=enabled
+%else
+    %define arg_nbdkit_config_default -Dnbdkit_config_default=disabled
+%endif
+
 %if %{with_fuse}
     %define arg_fuse -Dfuse=enabled
 %else
@@ -1322,6 +1331,7 @@ export SOURCE_DATE_EPOCH=$(stat --printf='%Y' %{_specdir}/libvirt.spec)
            %{?arg_sanlock} \
            -Dlibpcap=enabled \
            %{?arg_nbdkit} \
+           %{?arg_nbdkit_config_default} \
            -Dlibnl=enabled \
            -Daudit=enabled \
            -Ddtrace=enabled \

meson.build

@@ -1009,6 +1009,16 @@ if not conf.has('WITH_NBDKIT')
   libnbd_dep = dependency('', required: false)
 endif
 
+# default value for storage_use_nbdkit config option.
+# For now 'auto' just maps to disabled, but in the future it may depend on
+# which security drivers are enabled
+use_nbdkit_default = get_option('nbdkit_config_default').enabled()
+
+if use_nbdkit_default and not conf.has('WITH_NBDKIT')
+  error('nbdkit_config_default requires nbdkit to be enabled')
+endif
+conf.set10('USE_NBDKIT_DEFAULT', use_nbdkit_default)
+
 libnl_version = '3.0'
 if not get_option('libnl').disabled() and host_machine.system() == 'linux'
   libnl_dep = dependency('libnl-3.0', version: '>=' + libnl_version, required: get_option('libnl'))

meson_options.txt

@@ -104,7 +104,8 @@ option('loader_nvram', type: 'string', value: '', description: 'Pass list of pai
 option('login_shell', type: 'feature', value: 'auto', description: 'build virt-login-shell')
 option('nss', type: 'feature', value: 'auto', description: 'enable Name Service Switch plugin for resolving guest IP addresses')
 option('numad', type: 'feature', value: 'auto', description: 'use numad to manage CPU placement dynamically')
-option('nbdkit', type: 'feature', value: 'auto', description: 'use nbdkit to access network disks')
+option('nbdkit', type: 'feature', value: 'auto', description: 'Build nbdkit storage backend')
+option('nbdkit_config_default', type: 'feature', value: 'auto', description: 'Whether to use nbdkit storage backend for network disks by default (configurable)')
 option('pm_utils', type: 'feature', value: 'auto', description: 'use pm-utils for power management')
 option('sysctl_config', type: 'feature', value: 'auto', description: 'Whether to install sysctl configs')
 option('tls_priority', type: 'string', value: 'NORMAL', description: 'set the default TLS session priority string')

src/qemu/libvirtd_qemu.aug

@@ -147,6 +147,8 @@ module Libvirtd_qemu =
 
    let capability_filters_entry = str_array_entry "capability_filters"
 
+   let storage_entry = bool_entry "storage_use_nbdkit"
+
    (* Each entry in the config is one of the following ... *)
    let entry = default_tls_entry
              | vnc_entry
@@ -170,6 +172,7 @@ module Libvirtd_qemu =
              | nbd_entry
              | swtpm_entry
              | capability_filters_entry
+             | storage_entry
              | obsolete_entry
 
    let comment = [ label "#comment" . del /#[ \t]*/ "# " .  store /([^ \t\n][^\n]*)?/ . del /\n/ "\n" ]

src/qemu/meson.build

@@ -137,6 +137,7 @@ if conf.has('WITH_QEMU')
   qemu_user_group_conf = configuration_data({
     'QEMU_USER': qemu_user,
     'QEMU_GROUP': qemu_group,
+    'USE_NBDKIT_DEFAULT': use_nbdkit_default.to_int(),
   })
   qemu_conf = configure_file(
     input: 'qemu.conf.in',
@@ -147,6 +148,7 @@ if conf.has('WITH_QEMU')
   qemu_user_group_hack_conf = configuration_data({
     'QEMU_USER': qemu_user,
     'QEMU_GROUP': qemu_group,
+    'USE_NBDKIT_DEFAULT': use_nbdkit_default.to_int(),
     # This hack is necessary because the output file is going to be
     # used as input for another configure_file() call later, which
     # will take care of substituting @CONFIG@ with useful data

src/qemu/qemu.conf.in

@@ -974,3 +974,14 @@
 # "full"    -  both QEMU and its helper processes are placed into separate
 #              scheduling group
 #sched_core = "none"
+
+# Using nbdkit to access remote disk sources
+#
+# If this is set then libvirt will use nbdkit to access remote disk sources
+# when available. nbdkit will export an NBD share to QEMU rather than having
+# QEMU attempt to access the remote server directly.
+#
+# Possible values are 0 or 1. Default value is @USE_NBDKIT_DEFAULT@. Please
+# note that the default might change in future releases.
+#
+#storage_use_nbdkit = @USE_NBDKIT_DEFAULT@

src/qemu/qemu_conf.c

@@ -285,6 +285,7 @@ virQEMUDriverConfig *virQEMUDriverConfigNew(bool privileged,
         return NULL;
 
     cfg->deprecationBehavior = g_strdup("none");
+    cfg->storageUseNbdkit = USE_NBDKIT_DEFAULT;
 
     return g_steal_pointer(&cfg);
 }
@@ -1065,6 +1066,24 @@ virQEMUDriverConfigLoadCapsFiltersEntry(virQEMUDriverConfig *cfg,
 }
 
 
+static int
+virQEMUDriverConfigLoadStorageEntry(virQEMUDriverConfig *cfg,
+                                    virConf *conf)
+{
+    if (virConfGetValueBool(conf, "storage_use_nbdkit", &cfg->storageUseNbdkit) < 0)
+        return -1;
+
+#if !WITH_NBDKIT
+    if (cfg->storageUseNbdkit) {
+        VIR_WARN("Ignoring configuration option 'storage_use_nbdkit': nbdkit is not supported by this libvirt");
+        cfg->storageUseNbdkit = false;
+    }
+#endif /* WITH_NBDKIT */
+
+    return 0;
+}
+
+
 int virQEMUDriverConfigLoadFile(virQEMUDriverConfig *cfg,
                                 const char *filename,
                                 bool privileged)
@@ -1136,6 +1155,9 @@ int virQEMUDriverConfigLoadFile(virQEMUDriverConfig *cfg,
     if (virQEMUDriverConfigLoadCapsFiltersEntry(cfg, conf) < 0)
         return -1;
 
+    if (virQEMUDriverConfigLoadStorageEntry(cfg, conf) < 0)
+        return -1;
+
     return 0;
 }
 

src/qemu/qemu_conf.h

@@ -230,6 +230,8 @@ struct _virQEMUDriverConfig {
 
     char *deprecationBehavior;
 
+    bool storageUseNbdkit;
+
     virQEMUSchedCore schedCore;
 };
 

src/qemu/qemu_domain.c

@@ -10296,6 +10296,9 @@ qemuDomainPrepareStorageSourceNbdkit(virStorageSource *src,
 {
     g_autoptr(qemuNbdkitCaps) nbdkit = NULL;
 
+    if (!cfg->storageUseNbdkit)
+        return false;
+
     if (virStorageSourceGetActualType(src) != VIR_STORAGE_TYPE_NETWORK)
         return false;
 

src/qemu/test_libvirtd_qemu.aug.in

@@ -117,3 +117,4 @@ module Test_libvirtd_qemu =
 }
 { "deprecation_behavior" = "none" }
 { "sched_core" = "none" }
+{ "storage_use_nbdkit" = "@USE_NBDKIT_DEFAULT@" }

tests/qemuxml2argvtest.c

@@ -1125,7 +1125,6 @@ mymain(void)
     DO_TEST_CAPS_LATEST("disk-cdrom-empty-network-invalid");
     DO_TEST_CAPS_LATEST("disk-cdrom-bus-other");
     DO_TEST_CAPS_LATEST("disk-cdrom-network");
-    DO_TEST_CAPS_LATEST_NBDKIT("disk-cdrom-network-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
     DO_TEST_CAPS_LATEST("disk-cdrom-tray");
     DO_TEST_CAPS_LATEST("disk-floppy");
     DO_TEST_CAPS_LATEST("disk-floppy-q35");
@@ -1171,8 +1170,6 @@ mymain(void)
     DO_TEST_CAPS_VER("disk-network-sheepdog", "6.0.0");
     DO_TEST_CAPS_LATEST("disk-network-source-auth");
     DO_TEST_CAPS_LATEST("disk-network-source-curl");
-    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-source-curl-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
-    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-source-curl-nbdkit-backing", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
     DO_TEST_CAPS_LATEST("disk-network-nfs");
     driver.config->vxhsTLS = 1;
     driver.config->nbdTLSx509secretUUID = g_strdup("6fd3f62d-9fe7-4a4e-a869-7acd6376d8ea");
@@ -1183,13 +1180,10 @@ mymain(void)
     DO_TEST_CAPS_LATEST("disk-network-tlsx509-nbd-hostname");
     DO_TEST_CAPS_VER("disk-network-tlsx509-vxhs", "5.0.0");
     DO_TEST_CAPS_LATEST("disk-network-http");
-    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-http-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
     VIR_FREE(driver.config->nbdTLSx509secretUUID);
     VIR_FREE(driver.config->vxhsTLSx509secretUUID);
     driver.config->vxhsTLS = 0;
     DO_TEST_CAPS_LATEST("disk-network-ssh");
-    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-ssh-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_SSH);
-    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-ssh-password", QEMU_NBDKIT_CAPS_PLUGIN_SSH);
     DO_TEST_CAPS_LATEST("disk-no-boot");
     DO_TEST_CAPS_LATEST("disk-nvme");
     DO_TEST_CAPS_VER("disk-vhostuser-numa", "4.2.0");
@@ -1259,6 +1253,15 @@ mymain(void)
     DO_TEST_CAPS_LATEST("disk-geometry");
     DO_TEST_CAPS_LATEST("disk-blockio");
 
+    driver.config->storageUseNbdkit = 1;
+    DO_TEST_CAPS_LATEST_NBDKIT("disk-cdrom-network-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
+    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-source-curl-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
+    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-source-curl-nbdkit-backing", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
+    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-http-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_CURL);
+    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-ssh-nbdkit", QEMU_NBDKIT_CAPS_PLUGIN_SSH);
+    DO_TEST_CAPS_LATEST_NBDKIT("disk-network-ssh-password", QEMU_NBDKIT_CAPS_PLUGIN_SSH);
+    driver.config->storageUseNbdkit = 0;
+
     DO_TEST_CAPS_VER("disk-virtio-scsi-reservations", "5.2.0");
     DO_TEST_CAPS_LATEST("disk-virtio-scsi-reservations");