[PATCH v2] virtio: skip legacy support check on machine types less than 5.1

[Stefano Garzarella <sgarzare@redhat.com>]

Commit 9b3a35ec82 ("virtio: verify that legacy support is not accidentally
on") added a check that returns an error if legacy support is on, but the
device is not legacy.

Unfortunately some devices were wrongly declared legacy even if they
were not (e.g vhost-vsock).

To avoid migration issues, we disable this error for machine types < 5.1,
but we print a warning.

Cc: qemu-stable@nongnu.org
Fixes: 9b3a35ec82 ("virtio: verify that legacy support is not accidentally on")
Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Suggested-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
v2:
 - fixed Cornelia's e-mail address
---
 include/hw/virtio/virtio.h |  1 +
 hw/core/machine.c          |  1 +
 hw/virtio/virtio.c         | 18 ++++++++++++++++--
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 807280451b..ed7cee348b 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -103,6 +103,7 @@ struct VirtIODevice
     bool use_started;
     bool started;
     bool start_on_kick; /* when virtio 1.0 feature has not been negotiated */
+    bool disable_legacy_check;
     VMChangeStateEntry *vmstate;
     char *bus_name;
     uint8_t device_endian;
diff --git a/hw/core/machine.c b/hw/core/machine.c
index ea26d61237..b686eab798 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -44,6 +44,7 @@ GlobalProperty hw_compat_5_0[] = {
     { "vmport", "x-signal-unsupported-cmd", "off" },
     { "vmport", "x-report-vmx-type", "off" },
     { "vmport", "x-cmds-v2", "off" },
+    { "virtio-device", "x-disable-legacy-check", "true" },
 };
 const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
 
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index e983025217..30ccc43b8c 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3287,6 +3287,8 @@ void virtio_init(VirtIODevice *vdev, const char *name,
  */
 bool virtio_legacy_allowed(VirtIODevice *vdev)
 {
+    bool ret = false;
+
     switch (vdev->device_id) {
     case VIRTIO_ID_NET:
     case VIRTIO_ID_BLOCK:
@@ -3298,10 +3300,20 @@ bool virtio_legacy_allowed(VirtIODevice *vdev)
     case VIRTIO_ID_9P:
     case VIRTIO_ID_RPROC_SERIAL:
     case VIRTIO_ID_CAIF:
+        ret = true;
+    }
+
+    /*
+     * For backward compatibility, we allow legacy mode with old machine types
+     * to get the migration working.
+     */
+    if (!ret && vdev->disable_legacy_check) {
+        warn_report("device is modern-only, but for backward compatibility "
+                    "legacy is allowed");
         return true;
-    default:
-        return false;
     }
+
+    return ret;
 }
 
 hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n)
@@ -3713,6 +3725,8 @@ static Property virtio_properties[] = {
     DEFINE_VIRTIO_COMMON_FEATURES(VirtIODevice, host_features),
     DEFINE_PROP_BOOL("use-started", VirtIODevice, use_started, true),
     DEFINE_PROP_BOOL("use-disabled-flag", VirtIODevice, use_disabled_flag, true),
+    DEFINE_PROP_BOOL("x-disable-legacy-check", VirtIODevice,
+                     disable_legacy_check, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.26.2