Re: [Qemu-devel] [2.5 issue] virtio-1 in virtio-net and old vhost

[Cornelia Huck]

On Tue, 1 Dec 2015 11:11:08 +0100
Cornelia Huck <address@hidden> wrote:

> Some of our test folks tried to run a recent-ish qemu (nearly 2.5)
> combined with an old host kernel (and a virtio-1 capable guest).
> 
> In that setup, we had the transport (in that case, virtio-ccw)
> advertise VERSION_1 as it is a revision 1 device. However, the old
> vhost driver did not support virtio-1 and therefore cleared the
> VERSION_1 bit. In the end, qemu did not offer VERSION_1 to the guest
> for a revision 1 device, which the guest treats as a fatal error.
> 
> It looks to me as if virtio-pci has the same problem: The kernel will
> detect a modern device as by the I/O layout and then barf at the
> missing VERSION_1 feature bit.
> 
> We _could_ make this missing VERSION_1 bit non-fatal in the guest, but
> that does not fix guests that are already out there.
> 
> The problem is that the transport cannot know whether the VERSION_1 bit
> will be pulled from under it later during device setup: This is only
> done in the ->get_features() callback when virtio-net will handle the
> features supported by vhost.
> 
> I'm currently lacking a good idea on how to fix this, but I think that
> is an issue that should be dealt with for 2.5...

What about the following (completely untested)? Have the transport
verify that VERSION_1 is still present after get_features. Should do
for virtio-ccw, but I'm not sure whether virtio-pci can be unrolled in
that way.

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index fb103b7..87ecbc1 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -1555,6 +1555,16 @@ static void virtio_ccw_device_plugged(DeviceState *d, 
Error **errp)
                           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)) {
+       dev->max_rev = 0;
+   }
+}
+
 static void virtio_ccw_device_unplugged(DeviceState *d)
 {
     VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
@@ -1891,6 +1901,7 @@ static void virtio_ccw_bus_class_init(ObjectClass *klass, 
void *data)
     k->save_config = virtio_ccw_save_config;
     k->load_config = virtio_ccw_load_config;
     k->device_plugged = virtio_ccw_device_plugged;
+    k->post_plugged = virtio_ccw_post_plugged;
     k->device_unplugged = virtio_ccw_device_unplugged;
 }
 
diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index febda76..81c7cdd 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -56,6 +56,9 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
**errp)
     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);
+    }
 }
 
 /* Reset the virtio_bus */
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index dd48562..06e449c 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1741,6 +1741,30 @@ static void virtio_pci_device_plugged(DeviceState *d, 
Error **errp)
     virtio_add_feature(&vdev->host_features, VIRTIO_F_BAD_FEATURE);
 }
 
+static void virtio_pci_post_plugged(DeviceState *d, Error **errp)
+{
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
+    bool legacy = !(proxy->flags & VIRTIO_PCI_FLAG_DISABLE_LEGACY);
+    bool modern = !(proxy->flags & VIRTIO_PCI_FLAG_DISABLE_MODERN);
+    bool modern_pio = proxy->flags & VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY;
+    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+
+    if (modern && !virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1)) {
+        if (legacy) {
+            virtio_pci_modern_mem_region_unmap(proxy, &proxy->common);
+            virtio_pci_modern_mem_region_unmap(proxy, &proxy->isr);
+            virtio_pci_modern_mem_region_unmap(proxy, &proxy->device);
+            virtio_pci_modern_mem_region_unmap(proxy, &proxy->notify);
+            if (modern_pio) {
+                virtio_pci_modern_io_region_unmap(proxy, &proxy->notify_pio);
+            }
+            proxy->flags |= VIRTIO_PCI_FLAG_DISABLE_MODERN;
+        } else {
+            error_setg(errp, "can't fall back to legacy virtio");
+        }
+    }
+}
+
 static void virtio_pci_device_unplugged(DeviceState *d)
 {
     VirtIOPCIProxy *proxy = VIRTIO_PCI(d);
@@ -2474,6 +2498,7 @@ static void virtio_pci_bus_class_init(ObjectClass *klass, 
void *data)
     k->set_guest_notifiers = virtio_pci_set_guest_notifiers;
     k->vmstate_change = virtio_pci_vmstate_change;
     k->device_plugged = virtio_pci_device_plugged;
+    k->post_plugged = virtio_pci_post_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 6c3d4cb..ff0a3b0 100644
--- a/include/hw/virtio/virtio-bus.h
+++ b/include/hw/virtio/virtio-bus.h
@@ -59,6 +59,7 @@ typedef struct VirtioBusClass {
      * This is called by virtio-bus just after the device is plugged.
      */
     void (*device_plugged)(DeviceState *d, Error **errp);
+    void (*post_plugged)(DeviceState *d, Error **errp);
     /*
      * transport independent exit function.
      * This is called by virtio-bus just before the device is unplugged.