[Qemu-devel] [RFC] virtio-bus: Plug devices after features are negotiated

[Maxime Coquelin]

Currently, devices are plugged before features are negotiated.
If the backend doesn't support VIRTIO_F_VERSION_1, the transport
need to rewind some settings.

This is the case for both PCI and CCW.
For CCW, a post_plugged callback had been introduced, where
max_rev field is just updated if VIRTIO_F_VERSION_1 is not
supported by the backend.
For PCI, implementing the post_plugged would be much more
complicated, so the current fix consists in checking whether
the backend supports VIRTIO_F_VERSION_1 in the backend.

This patch propose to replace existing solutions with a common
approach by negociating features before ->device_plugged() is
called. A pre_plugged callback is introduced so that the
transports can set their supported features.

Cc: Cornelia Huck <address@hidden>
Cc: Marcel Apfelbaum <address@hidden>
Cc: Michael S. Tsirkin <address@hidden>
Signed-off-by: Maxime Coquelin <address@hidden>
---
Cornelia,

This is what I proposed earlier today.

I have tested it successfully with PCI.
Does it sound good to you? Would you have a try with CCW?

Note that this is based on my earlier series which fixes PCI.

Thanks,
Maxime
---
 hw/s390x/virtio-ccw.c          | 30 +++++++++++++++---------------
 hw/virtio/virtio-bus.c         |  9 +++++----
 hw/virtio/virtio-pci.c         | 18 ++++++++++++++----
 include/hw/virtio/virtio-bus.h | 10 +++++-----
 4 files changed, 39 insertions(+), 28 deletions(-)

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index a554a24..e10a88d 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -1261,6 +1261,16 @@ static int virtio_ccw_load_config(DeviceState *d, 
QEMUFile *f)
     return 0;
 }
 
+static void virtio_ccw_pre_plugged(DeviceState *d, Error **errp)
+{
+   VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
+   VirtIODevice *vdev = virtio_bus_get_device(&dev->bus);
+
+    if (dev->max_rev >= 1) {
+        virtio_add_feature(&vdev->host_features, VIRTIO_F_VERSION_1);
+    }
+}
+
 /* This is called by virtio-bus just after the device is plugged. */
 static void virtio_ccw_device_plugged(DeviceState *d, Error **errp)
 {
@@ -1270,6 +1280,10 @@ static void virtio_ccw_device_plugged(DeviceState *d, 
Error **errp)
     SubchDev *sch = ccw_dev->sch;
     int n = virtio_get_num_queues(vdev);
 
+    if (!virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
+        dev->max_rev = 0;
+    }
+
     if (virtio_get_num_queues(vdev) > VIRTIO_CCW_QUEUE_MAX) {
         error_setg(errp, "The number of virtqueues %d "
                    "exceeds ccw limit %d", n,
@@ -1283,25 +1297,11 @@ static void virtio_ccw_device_plugged(DeviceState *d, 
Error **errp)
 
     sch->id.cu_model = virtio_bus_get_vdev_id(&dev->bus);
 
-    if (dev->max_rev >= 1) {
-        virtio_add_feature(&vdev->host_features, VIRTIO_F_VERSION_1);
-    }
 
     css_generate_sch_crws(sch->cssid, sch->ssid, sch->schid,
                           d->hotplugged, 1);
 }
 
-static void virtio_ccw_post_plugged(DeviceState *d, Error **errp)
-{
-   VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
-   VirtIODevice *vdev = virtio_bus_get_device(&dev->bus);
-
-   if (!virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1)) {
-        /* A backend didn't support modern virtio. */
-       dev->max_rev = 0;
-   }
-}
-
 static void virtio_ccw_device_unplugged(DeviceState *d)
 {
     VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
@@ -1593,8 +1593,8 @@ static void virtio_ccw_bus_class_init(ObjectClass *klass, 
void *data)
     k->load_queue = virtio_ccw_load_queue;
     k->save_config = virtio_ccw_save_config;
     k->load_config = virtio_ccw_load_config;
+    k->pre_plugged = virtio_ccw_pre_plugged;
     k->device_plugged = virtio_ccw_device_plugged;
-    k->post_plugged = virtio_ccw_post_plugged;
     k->device_unplugged = virtio_ccw_device_unplugged;
     k->ioeventfd_started = virtio_ccw_ioeventfd_started;
     k->ioeventfd_set_started = virtio_ccw_ioeventfd_set_started;
diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index a85b7c8..9d2111f 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -49,16 +49,17 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
**errp)
 
     DPRINTF("%s: plug device.\n", qbus->name);
 
-    if (klass->device_plugged != NULL) {
-        klass->device_plugged(qbus->parent, errp);
+    if (klass->pre_plugged != NULL) {
+        klass->pre_plugged(qbus->parent, errp);
     }
 
     /* Get the features of the plugged device. */
     assert(vdc->get_features != NULL);
     vdev->host_features = vdc->get_features(vdev, vdev->host_features,
                                             errp);
-    if (klass->post_plugged != NULL) {
-        klass->post_plugged(qbus->parent, errp);
+
+    if (klass->device_plugged != NULL) {
+        klass->device_plugged(qbus->parent, errp);
     }
 }
 
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 9e88d7b..07e6c60 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1569,6 +1569,18 @@ static void 
virtio_pci_modern_io_region_unmap(VirtIOPCIProxy *proxy,
                                 &region->mr);
 }
 
+static void virtio_pci_pre_plugged(DeviceState *d, Error **errp)
+{
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
+    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+
+    if (virtio_pci_modern(proxy)) {
+        virtio_add_feature(&vdev->host_features, VIRTIO_F_VERSION_1);
+    }
+
+    virtio_add_feature(&vdev->host_features, VIRTIO_F_BAD_FEATURE);
+}
+
 /* This is called by virtio-bus just after the device is plugged. */
 static void virtio_pci_device_plugged(DeviceState *d, Error **errp)
 {
@@ -1585,7 +1597,7 @@ static void virtio_pci_device_plugged(DeviceState *d, 
Error **errp)
      * Virtio capabilities present without
      * VIRTIO_F_VERSION_1 confuses guests
      */
-    if (!virtio_test_backend_virtio_1(vdev, errp)) {
+    if (!virtio_has_feature(vdev->host_features, VIRTIO_F_VERSION_1)) {
         virtio_pci_disable_modern(proxy);
     }
 
@@ -1637,7 +1649,6 @@ static void virtio_pci_device_plugged(DeviceState *d, 
Error **errp)
 
         struct virtio_pci_cfg_cap *cfg_mask;
 
-        virtio_add_feature(&vdev->host_features, VIRTIO_F_VERSION_1);
         virtio_pci_modern_regions_init(proxy);
 
         virtio_pci_modern_mem_region_map(proxy, &proxy->common, &cap);
@@ -1702,8 +1713,6 @@ static void virtio_pci_device_plugged(DeviceState *d, 
Error **errp)
     if (!kvm_has_many_ioeventfds()) {
         proxy->flags &= ~VIRTIO_PCI_FLAG_USE_IOEVENTFD;
     }
-
-    virtio_add_feature(&vdev->host_features, VIRTIO_F_BAD_FEATURE);
 }
 
 static void virtio_pci_device_unplugged(DeviceState *d)
@@ -2459,6 +2468,7 @@ static void virtio_pci_bus_class_init(ObjectClass *klass, 
void *data)
     k->query_guest_notifiers = virtio_pci_query_guest_notifiers;
     k->set_guest_notifiers = virtio_pci_set_guest_notifiers;
     k->vmstate_change = virtio_pci_vmstate_change;
+    k->pre_plugged = virtio_pci_pre_plugged;
     k->device_plugged = virtio_pci_device_plugged;
     k->device_unplugged = virtio_pci_device_unplugged;
     k->query_nvectors = virtio_pci_query_nvectors;
diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
index f3e5ef3..24caa0a 100644
--- a/include/hw/virtio/virtio-bus.h
+++ b/include/hw/virtio/virtio-bus.h
@@ -54,16 +54,16 @@ typedef struct VirtioBusClass {
     int (*set_guest_notifiers)(DeviceState *d, int nvqs, bool assign);
     void (*vmstate_change)(DeviceState *d, bool running);
     /*
+     * Expose the features the transport layer supports before
+     * the negotiation takes place.
+     */
+    void (*pre_plugged)(DeviceState *d, Error **errp);
+    /*
      * transport independent init function.
      * This is called by virtio-bus just after the device is plugged.
      */
     void (*device_plugged)(DeviceState *d, Error **errp);
     /*
-     * Re-evaluate setup after feature bits have been validated
-     * by the device backend.
-     */
-    void (*post_plugged)(DeviceState *d, Error **errp);
-    /*
      * transport independent exit function.
      * This is called by virtio-bus just before the device is unplugged.
      */
-- 
2.7.4