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

[Cornelia Huck]

On Thu, 13 Aug 2020 11:16:56 +0200
Stefano Garzarella <sgarzare@redhat.com> wrote:

> 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")

Oh, I had heard that from others already, was still trying to figure
out what to do.

> 
> 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?

We basically have three possible ways to deal with this:

- Force it to modern (i.e., what you have been doing; would need the
  equivalent changes in ccw as well.)
  Pro: looks like the cleanest approach.
  Con: not sure if we would need backwards compatibility support,
  which looks hairy.
- Add vsock to the list of devices with legacy support.
  Pro: Existing setups continue to work.
  Con: If vsock is really virtio-1-only, we still carry around
  possibly broken legacy support.
- Do nothing, have users force legacy off. Bad idea, as ccw has no way
  to do that on the command line.

The first option is probably best.