On Thu, 13 Aug 2020 12:24:30 +0200
Stefano Garzarella <sgarz...@redhat.com> wrote:
> On Thu, Aug 13, 2020 at 11:28:20AM +0200, Cornelia Huck wrote:
> > On Thu, 13 Aug 2020 11:16:56 +0200
> > Stefano Garzarella <sgarz...@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?
No, ->max_rev is the wrong side of the limit :) You want
ccw_dev->force_revision_1 = true;
in _instance_init() (see e.g. virtio-ccw-gpu.c).
>
> > Pro: looks like the cleanest approach.
> > Con: not sure if we would need backwards compatibility support,
> > which looks hairy.
>
> Not sure too.
Yes, I'm not sure at all how to handle user-specified values for
legacy/modern.
>
> > - 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.
Yes, I would guess so as well.
>
> How can I verify that? Maybe forcing legacy mode and run some tests.
Probably yes. The likeliest area with issues is probably endianness, so
maybe with something big endian in the mix?
>
> > - 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!
Yes, it's really a pity we only noticed this after the release; this
was supposed to stop new devices with legacy support creeping in, not
to break existing command lines :(
