qemu: remove default audio backend for migratable XML

When seeing a guest with a sound device, and no audio backend, we
automatically add an audio backend XML element based on the historical
QEMU driver behaviour. Unfortunately when we live migrate back to an
old libvirt, it may not understand the audio driver type we configured.
We thus need to strip the default audio backend when migrating.

Fixes https://gitlab.com/libvirt/libvirt/-/issues/179
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/libvirt_private.syms

@@ -226,6 +226,7 @@ virDiskNameParse;
 virDiskNameToBusDeviceIndex;
 virDiskNameToIndex;
 virDomainActualNetDefFree;
+virDomainAudioDefFree;
 virDomainAudioFormatTypeFromString;
 virDomainAudioFormatTypeToString;
 virDomainAudioIOCommonIsSet;

src/qemu/qemu_domain.c

@@ -3587,18 +3587,19 @@ qemuDomainDefAddImplicitInputDevice(virDomainDef *def)
 }
 
 static int
-qemuDomainDefAddDefaultAudioBackend(virQEMUDriver *driver,
-                                    virDomainDef *def)
+qemuDomainDefSuggestDefaultAudioBackend(virQEMUDriver *driver,
+                                        virDomainDef *def,
+                                        bool *addAudio,
+                                        int *audioBackend,
+                                        int *audioSDLDriver)
 {
     size_t i;
-    bool addAudio = false;
     bool audioPassthrough = false;
-    int audioBackend = VIR_DOMAIN_AUDIO_TYPE_NONE;
     g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
 
-    if (def->naudios > 0) {
-        return 0;
-    }
+    *addAudio = false;
+    *audioBackend = VIR_DOMAIN_AUDIO_TYPE_NONE;
+    *audioSDLDriver = VIR_DOMAIN_AUDIO_SDL_DRIVER_DEFAULT;
 
     for (i = 0; i < def->ngraphics; i++) {
         virDomainGraphicsDef *graph = def->graphics[i];
@@ -3609,18 +3610,18 @@ qemuDomainDefAddDefaultAudioBackend(virQEMUDriver *driver,
                 audioPassthrough = true;
             } else {
                 audioPassthrough = false;
-                audioBackend = VIR_DOMAIN_AUDIO_TYPE_NONE;
+                *audioBackend = VIR_DOMAIN_AUDIO_TYPE_NONE;
             }
-            addAudio = true;
+            *addAudio = true;
             break;
         case VIR_DOMAIN_GRAPHICS_TYPE_SPICE:
             audioPassthrough = false;
-            audioBackend = VIR_DOMAIN_AUDIO_TYPE_SPICE;
-            addAudio = true;
+            *audioBackend = VIR_DOMAIN_AUDIO_TYPE_SPICE;
+            *addAudio = true;
             break;
         case VIR_DOMAIN_GRAPHICS_TYPE_SDL:
             audioPassthrough = true;
-            addAudio = true;
+            *addAudio = true;
             break;
 
         case VIR_DOMAIN_GRAPHICS_TYPE_RDP:
@@ -3639,24 +3640,24 @@ qemuDomainDefAddDefaultAudioBackend(virQEMUDriver *driver,
             audioPassthrough = true;
         } else {
             audioPassthrough = false;
-            audioBackend = VIR_DOMAIN_AUDIO_TYPE_NONE;
+            *audioBackend = VIR_DOMAIN_AUDIO_TYPE_NONE;
         }
-        addAudio = true;
+        *addAudio = true;
     }
 
-    if (addAudio && audioPassthrough) {
+    if (*addAudio && audioPassthrough) {
         const char *audioenv = g_getenv("QEMU_AUDIO_DRV");
         if (audioenv == NULL) {
-            addAudio = false;
+            *addAudio = false;
         } else {
             /*
              * QEMU audio driver names are mostly the same as
              * libvirt XML audio backend names
              */
             if (STREQ(audioenv, "pa")) {
-                audioBackend = VIR_DOMAIN_AUDIO_TYPE_PULSEAUDIO;
+                *audioBackend = VIR_DOMAIN_AUDIO_TYPE_PULSEAUDIO;
             } else {
-                if ((audioBackend = virDomainAudioTypeTypeFromString(audioenv)) < 0) {
+                if (((*audioBackend) = virDomainAudioTypeTypeFromString(audioenv)) < 0) {
                     virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                                    _("unknown QEMU_AUDIO_DRV setting %s"), audioenv);
                     return -1;
@@ -3664,6 +3665,79 @@ qemuDomainDefAddDefaultAudioBackend(virQEMUDriver *driver,
             }
         }
     }
+
+    if (*addAudio && *audioBackend == VIR_DOMAIN_AUDIO_TYPE_SDL) {
+        const char *sdldriver = g_getenv("SDL_AUDIODRIVER");
+        if (sdldriver != NULL &&
+            (((*audioSDLDriver) =
+              virDomainAudioSDLDriverTypeFromString(sdldriver)) <= 0)) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("unknown SDL_AUDIODRIVER setting %s"), sdldriver);
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
+static int
+qemuDomainDefClearDefaultAudioBackend(virQEMUDriver *driver,
+                                      virDomainDef *def)
+{
+    bool addAudio;
+    int audioBackend;
+    int audioSDLDriver;
+    virDomainAudioDef *audio;
+
+    if (def->naudios != 1) {
+        return 0;
+    }
+
+    if (qemuDomainDefSuggestDefaultAudioBackend(driver,
+                                                def,
+                                                &addAudio,
+                                                &audioBackend,
+                                                &audioSDLDriver) < 0)
+        return -1;
+
+    if (!addAudio)
+        return 0;
+
+    audio = def->audios[0];
+    if (audio->type != audioBackend)
+        return 0;
+
+    if (audio->type == VIR_DOMAIN_AUDIO_TYPE_SDL &&
+        audio->backend.sdl.driver != audioSDLDriver)
+        return 0;
+
+    virDomainAudioDefFree(audio);
+    g_free(def->audios);
+    def->naudios = 0;
+    def->audios = NULL;
+
+    return 0;
+}
+
+static int
+qemuDomainDefAddDefaultAudioBackend(virQEMUDriver *driver,
+                                    virDomainDef *def)
+{
+    bool addAudio;
+    int audioBackend;
+    int audioSDLDriver;
+
+    if (def->naudios > 0) {
+        return 0;
+    }
+
+    if (qemuDomainDefSuggestDefaultAudioBackend(driver,
+                                                def,
+                                                &addAudio,
+                                                &audioBackend,
+                                                &audioSDLDriver) < 0)
+        return -1;
+
     if (addAudio) {
         virDomainAudioDef *audio = g_new0(virDomainAudioDef, 1);
 
@@ -3674,16 +3748,8 @@ qemuDomainDefAddDefaultAudioBackend(virQEMUDriver *driver,
         def->audios = g_new0(virDomainAudioDef *, def->naudios);
         def->audios[0] = audio;
 
-        if (audioBackend == VIR_DOMAIN_AUDIO_TYPE_SDL) {
-            const char *sdldriver = g_getenv("SDL_AUDIODRIVER");
-            if (sdldriver != NULL &&
-                ((audio->backend.sdl.driver =
-                  virDomainAudioSDLDriverTypeFromString(sdldriver)) <= 0)) {
-                virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                               _("unknown SDL_AUDIODRIVER setting %s"), sdldriver);
-                return -1;
-            }
-        }
+        if (audioBackend == VIR_DOMAIN_AUDIO_TYPE_SDL)
+            audio->backend.sdl.driver = audioSDLDriver;
     }
 
     return 0;
@@ -6298,6 +6364,12 @@ qemuDomainDefFormatBufInternal(virQEMUDriver *driver,
 
         if (def->cpu && qemuDomainMakeCPUMigratable(def->cpu) < 0)
             return -1;
+
+        /* Old libvirt doesn't understand <audio> elements so
+         * remove any we added by default
+         */
+        if (qemuDomainDefClearDefaultAudioBackend(driver, def) < 0)
+            return -1;
     }
 
  format:

tests/qemumigrationcookiexmldata/full-xml2xml-out.xml

@@ -142,7 +142,6 @@
       <sound model='ich6'>
         <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
       </sound>
-      <audio id='1' type='spice'/>
       <video>
         <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
         <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>