Re: [PATCH RFC] virtio-fs: force virtio 1.x usage

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 29 Jun 2020 10:53:23 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Jun 29, 2020 at 12:27:58PM +0200, Cornelia Huck wrote:
> > virtio-fs devices are only specified for virtio-1, so it is unclear
> > how a legacy or transitional device should behave.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > ---
> > 
> > Forcing off legacy now (after the virtio-fs device has already been
> > available) may have unintended consequences, therefore RFC.
> > 
> > By default, a virtio-pci device uses 'AUTO' for disable_legacy, which
> > will resolve to different values based upon which bus the device has
> > been plugged. Therefore, forcing disable_legacy may result in the same
> > device or a quite different one.
> > 
> > Even though pre-virtio-1 behaviour of virtio-fs devices is simply not
> > specified, toggling disable_legacy will have implications for the BAR
> > layout, IIRC, and therefore a guest might end up getting a different
> > device, even if it always used it with virtio-1 anyway.
> > 
> > Not sure what the best way to solve this problem is. Adding a compat
> > property for disable_legacy=AUTO may be the right thing to do, but I'm
> > not quite clear if there are any further implications here.  
> 
> Well I notice that this device is not migrateable.
> So I think that we can just switch it over and be done with it.

Oh, that makes things easier. (I'm wondering if libvirt already
configures this correctly?)

> 
> 
> > Whatever we do here, we should make sure that the ccw incarnation of
> > this device indeed forces virtio-1.  
> 
> I agree. I notice that the API virtio_pci_force_virtio_1 turned out
> to be too fragile. I propose that instead we have a whitelist of
> devices which can be legacy or transitional. Force rest to modern.

Also, there are further complications because the mechanism is per
transport, and therefore easy to miss.

bool virtio_legacy_allowed(VirtIODevice *vdev)
{
    switch (vdev->device_id) {
    case <...>:
    <list of legacy-capable devices>
        return true;
    default:
        return false;
}

Seems straightforward enough.

> 
> 
> > ---
> >  hw/virtio/vhost-user-fs-pci.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/hw/virtio/vhost-user-fs-pci.c b/hw/virtio/vhost-user-fs-pci.c
> > index e11c889d82b3..244205edf765 100644
> > --- a/hw/virtio/vhost-user-fs-pci.c
> > +++ b/hw/virtio/vhost-user-fs-pci.c
> > @@ -44,6 +44,7 @@ static void vhost_user_fs_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
> >          vpci_dev->nvectors = dev->vdev.conf.num_request_queues + 2;
> >      }
> >  
> > +    virtio_pci_force_virtio_1(vpci_dev);
> >      qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
> >  }
> >  
> > -- 
> > 2.25.4  
>