hw/virtio/vhost-user: don't suppress F_CONFIG when supported

Previously we would silently suppress VHOST_USER_PROTOCOL_F_CONFIG
during the protocol negotiation if the QEMU stub hadn't implemented
the vhost_dev_config_notifier. However this isn't the only way we can
handle config messages, the existing vdc->get/set_config can do this
as well.

Lightly re-factor the code to check for both potential methods and
instead of silently squashing the feature error out. It is unlikely
that a vhost-user backend expecting to handle CONFIG messages will
behave correctly if they never get sent.

Fixes: 1c3e5a2 ("vhost-user: back SET/GET_CONFIG requests with a protocol feature")
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20220321153037.3622127-13-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/scsi/vhost-user-scsi.c

@@ -121,6 +121,7 @@ static void vhost_user_scsi_realize(DeviceState *dev, Error **errp)
     vsc->dev.backend_features = 0;
     vqs = vsc->dev.vqs;
 
+    s->vhost_user.supports_config = true;
     ret = vhost_dev_init(&vsc->dev, &s->vhost_user,
                          VHOST_BACKEND_TYPE_USER, 0, errp);
     if (ret < 0) {

hw/virtio/vhost-user.c

@@ -1949,14 +1949,15 @@ static int vhost_user_postcopy_notifier(NotifierWithReturn *notifier,
 static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
                                    Error **errp)
 {
-    uint64_t features, protocol_features, ram_slots;
+    uint64_t features, ram_slots;
     struct vhost_user *u;
+    VhostUserState *vus = (VhostUserState *) opaque;
     int err;
 
     assert(dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER);
 
     u = g_new0(struct vhost_user, 1);
-    u->user = opaque;
+    u->user = vus;
     u->dev = dev;
     dev->opaque = u;
 
@@ -1967,6 +1968,10 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
     }
 
     if (virtio_has_feature(features, VHOST_USER_F_PROTOCOL_FEATURES)) {
+        bool supports_f_config = vus->supports_config ||
+            (dev->config_ops && dev->config_ops->vhost_dev_config_notifier);
+        uint64_t protocol_features;
+
         dev->backend_features |= 1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
 
         err = vhost_user_get_u64(dev, VHOST_USER_GET_PROTOCOL_FEATURES,
@@ -1976,19 +1981,34 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
             return -EPROTO;
         }
 
-        dev->protocol_features =
-            protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
-
-        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
-            /* Don't acknowledge CONFIG feature if device doesn't support it */
-            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
-        } else if (!(protocol_features &
-                    (1ULL << VHOST_USER_PROTOCOL_F_CONFIG))) {
-            error_setg(errp, "Device expects VHOST_USER_PROTOCOL_F_CONFIG "
-                       "but backend does not support it.");
-            return -EINVAL;
+        /*
+         * We will use all the protocol features we support - although
+         * we suppress F_CONFIG if we know QEMUs internal code can not support
+         * it.
+         */
+        protocol_features &= VHOST_USER_PROTOCOL_FEATURE_MASK;
+
+        if (supports_f_config) {
+            if (!virtio_has_feature(protocol_features,
+                                    VHOST_USER_PROTOCOL_F_CONFIG)) {
+                error_setg(errp, "vhost-user device %s expecting "
+                           "VHOST_USER_PROTOCOL_F_CONFIG but the vhost-user backend does "
+                           "not support it.", dev->vdev->name);
+                return -EPROTO;
+            }
+        } else {
+            if (virtio_has_feature(protocol_features,
+                                   VHOST_USER_PROTOCOL_F_CONFIG)) {
+                warn_reportf_err(*errp, "vhost-user backend supports "
+                                 "VHOST_USER_PROTOCOL_F_CONFIG for "
+                                 "device %s but QEMU does not.",
+                                 dev->vdev->name);
+                protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
+            }
         }
 
+        /* final set of protocol features */
+        dev->protocol_features = protocol_features;
         err = vhost_user_set_protocol_features(dev, dev->protocol_features);
         if (err < 0) {
             error_setg_errno(errp, EPROTO, "vhost_backend_init failed");

include/hw/virtio/vhost-user.h

@@ -22,6 +22,7 @@ typedef struct VhostUserState {
     CharBackend *chr;
     VhostUserHostNotifier notifier[VIRTIO_QUEUE_MAX];
     int memory_slots;
+    bool supports_config;
 } VhostUserState;
 
 bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp);