Re: [RFC PATCH 04/27] vhost: add vhost_kernel_set_vring_enable

[Eugenio Perez Martin]

On Mon, Dec 7, 2020 at 5:43 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
>
> On Fri, Nov 20, 2020 at 07:50:42PM +0100, Eugenio Pérez wrote:
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  hw/virtio/vhost-backend.c | 29 +++++++++++++++++++++++++++++
> >  1 file changed, 29 insertions(+)
> >
> > diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
> > index 222bbcc62d..317f1f96fa 100644
> > --- a/hw/virtio/vhost-backend.c
> > +++ b/hw/virtio/vhost-backend.c
> > @@ -201,6 +201,34 @@ static int vhost_kernel_get_vq_index(struct vhost_dev 
> > *dev, int idx)
> >      return idx - dev->vq_index;
> >  }
> >
> > +static int vhost_kernel_set_vq_enable(struct vhost_dev *dev, unsigned idx,
> > +                                      bool enable)
> > +{
> > +    struct vhost_vring_file file = {
> > +        .index = idx,
> > +    };
> > +
> > +    if (!enable) {
> > +        file.fd = -1; /* Pass -1 to unbind from file. */
> > +    } else {
> > +        struct vhost_net *vn_dev = container_of(dev, struct vhost_net, 
> > dev);
> > +        file.fd = vn_dev->backend;
> > +    }
> > +
> > +    return vhost_kernel_net_set_backend(dev, &file);
>
> This is vhost-net specific even though the function appears to be
> generic. Is there a plan to extend this to all devices?
>

I expected each vhost backend to enable-disable in its own terms, but
I think it could be 100% virtio-device generic with something like the
device state capability:
https://lists.oasis-open.org/archives/virtio-comment/202012/msg00005.html
.

> > +}
> > +
> > +static int vhost_kernel_set_vring_enable(struct vhost_dev *dev, int enable)
> > +{
> > +    int i;
> > +
> > +    for (i = 0; i < dev->nvqs; ++i) {
> > +        vhost_kernel_set_vq_enable(dev, i, enable);
> > +    }
> > +
> > +    return 0;
> > +}
>
> I suggest exposing the per-vq interface (vhost_kernel_set_vq_enable())
> in VhostOps so it follows the ioctl interface.

It was actually the initial plan, I left as all-or-nothing to make less changes.

> vhost_kernel_set_vring_enable() can be moved to vhost.c can loop over
> all vqs if callers find it convenient to loop over all vqs.

I'm ok with it. Thinking out loud, I don't know if it is easier for
some devices to enable/disable all of it (less syscalls? less downtime
somehow?) but I find more generic and useful the per-virtqueue
approach.

Thanks!