virtio-vsock requires 'disable-legacy=on' in QEMU 5.1

[Stefano Garzarella]

Hi,

Qinghua discovered that virtio-vsock-pci requires 'disable-legacy=on' in
QEMU 5.1:
    $ ./qemu-system-x86_64 ... -device vhost-vsock-pci,guest-cid=5
    qemu-system-x86_64: -device vhost-vsock-pci,guest-cid=5:
    device is modern-only, use disable-legacy=on

Bisecting I found that this behaviour starts from this commit:
9b3a35ec82 ("virtio: verify that legacy support is not accidentally on")

IIUC virtio-vsock is modern-only, so I tried this patch and it works:

diff --git a/hw/virtio/vhost-user-vsock-pci.c b/hw/virtio/vhost-user-vsock-pci.c
index f4cf95873d..6e4cc874cd 100644
--- a/hw/virtio/vhost-user-vsock-pci.c
+++ b/hw/virtio/vhost-user-vsock-pci.c
@@ -40,6 +40,7 @@ static void vhost_user_vsock_pci_realize(VirtIOPCIProxy 
*vpci_dev, Error **errp)
     VHostUserVSockPCI *dev = VHOST_USER_VSOCK_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);

+    virtio_pci_force_virtio_1(vpci_dev);
     qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
 }

diff --git a/hw/virtio/vhost-vsock-pci.c b/hw/virtio/vhost-vsock-pci.c
index a815278e69..f641b974e9 100644
--- a/hw/virtio/vhost-vsock-pci.c
+++ b/hw/virtio/vhost-vsock-pci.c
@@ -44,6 +44,7 @@ static void vhost_vsock_pci_realize(VirtIOPCIProxy *vpci_dev, 
Error **errp)
     VHostVSockPCI *dev = VHOST_VSOCK_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);

+    virtio_pci_force_virtio_1(vpci_dev);
     qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
 }


Do you think this is the right approach or is there a better way to
solve this issue?

Thanks,
Stefano