qemu: Allow using regular audio backends with VNC

Currently all host audio backends are disabled if a VM is using VNC, in
favor of the QEMU VNC audio extension. Unfortunately no released VNC
client supports this extension, so users have no way of getting audio
to work if using VNC.

Add a new config option in qemu.conf which allows changing libvirt's
behavior, but keep the default intact.

v2: Fix doc typos, change name to vnc_allow_host_audio

src/qemu/libvirtd_qemu.aug

@@ -38,6 +38,7 @@ module Libvirtd_qemu =
                  | str_entry "save_image_format"
                  | str_entry "hugetlbfs_mount"
                  | bool_entry "relaxed_acs_check"
+                 | bool_entry "vnc_allow_host_audio"
 
    (* Each enty in the config is one of the following three ... *)
    let entry = vnc_entry

src/qemu/qemu.conf

@@ -168,3 +168,13 @@
 # be assigned to guests.
 #
 # relaxed_acs_check = 1
+
+
+# QEMU implements an extension for providing audio over a VNC connection,
+# though if your VNC client does not support it, your only chance for getting
+# sound output is through regular audio backends. By default, libvirt will
+# disable all QEMU sound backends if using VNC, since they can cause
+# permissions issues. Enabling this option will make libvirtd honor the
+# QEMU_AUDIO_DRV environment variable when using VNC.
+#
+# vnc_allow_host_audio = 0

src/qemu/qemu_conf.c

@@ -351,6 +351,10 @@ int qemudLoadDriverConfig(struct qemud_driver *driver,
     CHECK_TYPE ("relaxed_acs_check", VIR_CONF_LONG);
     if (p) driver->relaxedACS = p->l;
 
+    p = virConfGetValue (conf, "vnc_allow_host_audio");
+    CHECK_TYPE ("vnc_allow_host_audio", VIR_CONF_LONG);
+    if (p) driver->vncAllowHostAudio = p->l;
+
     virConfFree (conf);
     return 0;
 }
@@ -4399,12 +4403,15 @@ int qemudBuildCommandLine(virConnectPtr conn,
             ADD_ARG_LIT(def->graphics[0]->data.vnc.keymap);
         }
 
-        /* QEMU implements a VNC extension for providing audio, so we
-         * set the audio backend to none, to prevent it opening the
-         * host OS audio devices since that causes security issues
-         * and is non-sensical when using VNC.
+        /* Unless user requested it, set the audio backend to none, to
+         * prevent it opening the host OS audio devices, since that causes
+         * security issues and might not work when using VNC.
          */
+        if (driver->vncAllowHostAudio) {
+            ADD_ENV_COPY("QEMU_AUDIO_DRV");
+        } else {
             ADD_ENV_LIT("QEMU_AUDIO_DRV=none");
+        }
     } else if ((def->ngraphics == 1) &&
                def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_SDL) {
         char *xauth = NULL;

src/qemu/qemu_conf.h

@@ -138,6 +138,8 @@ struct qemud_driver {
 
     unsigned int relaxedACS : 1;
 
+    unsigned int vncAllowHostAudio : 1;
+
     virCapsPtr caps;
 
     /* An array of callbacks */

src/qemu/test_libvirtd_qemu.aug

@@ -97,6 +97,8 @@ save_image_format = \"gzip\"
 hugetlbfs_mount = \"/dev/hugepages\"
 
 relaxed_acs_check = 1
+
+vnc_allow_host_audio = 1
 "
 
    test Libvirtd_qemu.lns get conf =
@@ -204,3 +206,5 @@ relaxed_acs_check = 1
 { "hugetlbfs_mount" = "/dev/hugepages" }
 { "#empty" }
 { "relaxed_acs_check" = "1" }
+{ "#empty" }
+{ "vnc_allow_host_audio" = "1" }