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

[Stefano Garzarella]

On Thu, Aug 13, 2020 at 11:28:20AM +0200, Cornelia Huck wrote:
> 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.)

Oo, thanks for pointing out ccw!
I don't know ccw well, in this case should we set dev->max_rev to 1 or 2
to force to modern?

>   Pro: looks like the cleanest approach.
>   Con: not sure if we would need backwards compatibility support,
>   which looks hairy.

Not sure too.

> - 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.

I'm not sure it is virtio-1-only, but virtio-vsock was introduced in
2016, so I supposed it is modern-only.

How can I verify that? Maybe forcing legacy mode and run some tests.

> - 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.
>

Yeah, I agree with you!

Thanks,
Stefano