[PATCH v3 1/4] 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 add a virtio-device property
(x-disable-legacy-check) to skip the legacy error, printing a warning
instaed, for machine types < 5.1.

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>
---
v3:
 - added virtio_legacy_check_disabled() helper
 - moved warning where error was returned [Cornelia]
v2:
 - fixed Cornelia's e-mail address
---
 include/hw/virtio/virtio.h |  2 ++
 hw/core/machine.c          |  1 +
 hw/s390x/virtio-ccw.c      | 15 ++++++++++++---
 hw/virtio/virtio-pci.c     | 14 ++++++++++++--
 hw/virtio/virtio.c         |  7 +++++++
 5 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 807280451b..f90cfb03e3 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;
@@ -396,5 +397,6 @@ static inline bool virtio_device_disabled(VirtIODevice *vdev)
 }
 
 bool virtio_legacy_allowed(VirtIODevice *vdev);
+bool virtio_legacy_check_disabled(VirtIODevice *vdev);
 
 #endif
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/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index 8feb3451a0..97a01bc48a 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -1122,9 +1122,18 @@ static void virtio_ccw_device_plugged(DeviceState *d, Error **errp)
     }
 
     if (!virtio_ccw_rev_max(dev) && !virtio_legacy_allowed(vdev)) {
-        error_setg(errp, "Invalid value of property max_rev "
-                   "(is %d expected >= 1)", virtio_ccw_rev_max(dev));
-        return;
+        /*
+         * To avoid migration issues, we allow legacy mode when legacy
+         * check is disabled in the old machine types (< 5.1).
+         */
+        if (virtio_legacy_check_disabled(vdev)) {
+            warn_report("device is modern-only, but for backward "
+                        "compatibility legacy is allowed");
+        } else {
+            error_setg(errp, "Invalid value of property max_rev "
+                       "(is %d expected >= 1)", virtio_ccw_rev_max(dev));
+            return;
+        }
     }
 
     if (virtio_get_num_queues(vdev) > VIRTIO_QUEUE_MAX) {
diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 5bc769f685..bb91e34594 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -1597,8 +1597,18 @@ static void virtio_pci_device_plugged(DeviceState *d, Error **errp)
 
     if (legacy) {
         if (!virtio_legacy_allowed(vdev)) {
-            error_setg(errp, "device is modern-only, use disable-legacy=on");
-            return;
+            /*
+             * To avoid migration issues, we allow legacy mode when legacy
+             * check is disabled in the old machine types (< 5.1).
+             */
+            if (virtio_legacy_check_disabled(vdev)) {
+                warn_report("device is modern-only, but for backward "
+                            "compatibility legacy is allowed");
+            } else {
+                error_setg(errp,
+                           "device is modern-only, use disable-legacy=on");
+                return;
+            }
         }
         if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
             error_setg(errp, "VIRTIO_F_IOMMU_PLATFORM was supported by"
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index e983025217..b85277da67 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3304,6 +3304,11 @@ bool virtio_legacy_allowed(VirtIODevice *vdev)
     }
 }
 
+bool virtio_legacy_check_disabled(VirtIODevice *vdev)
+{
+    return vdev->disable_legacy_check;
+}
+
 hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n)
 {
     return vdev->vq[n].vring.desc;
@@ -3713,6 +3718,8 @@ 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