Subject: Re: [PATCH V4 net-next 3/3] virtio-net: synchronize operstate with admin state on up/down

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 1 Aug 2024 01:58:36 -0400

On Thu, Aug 01, 2024 at 10:16:00AM +0800, Jason Wang wrote:
> > > @@ -2885,6 +2886,25 @@ static void virtnet_cancel_dim(struct virtnet_info *vi, struct dim *dim)
> > >       net_dim_work_cancel(dim);
> > >  }
> > >
> > > +static void virtnet_update_settings(struct virtnet_info *vi)
> > > +{
> > > +     u32 speed;
> > > +     u8 duplex;
> > > +
> > > +     if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_SPEED_DUPLEX))
> > > +             return;
> > > +
> > > +     virtio_cread_le(vi->vdev, struct virtio_net_config, speed, &speed);
> > > +
> > > +     if (ethtool_validate_speed(speed))
> > > +             vi->speed = speed;
> > > +
> > > +     virtio_cread_le(vi->vdev, struct virtio_net_config, duplex, &duplex);
> > > +
> > > +     if (ethtool_validate_duplex(duplex))
> > > +             vi->duplex = duplex;
> > > +}
> > > +
> >
> > I already commented on this approach.  This is now invoked on each open,
> > lots of extra VM exits. No bueno, people are working hard to keep setup
> > overhead under control. Handle this in the config change interrupt -
> > your new infrastructure is perfect for this.
> 
> No, in this version it doesn't. Config space read only happens if
> there's a pending config interrupt during ndo_open:
> 
> +       if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
> +               if (vi->status & VIRTIO_NET_S_LINK_UP)
> +                       netif_carrier_on(vi->dev);
> +               virtio_config_driver_enable(vi->vdev);
> +       } else {
> +               vi->status = VIRTIO_NET_S_LINK_UP;
> +               netif_carrier_on(dev);
> +               virtnet_update_settings(vi);
> +       }

Sorry for being unclear, I was referring to !VIRTIO_NET_F_STATUS.
I do not see why do we need to bother re-reading settings in this case at all,
status is not there, nothing much changes.

> >
> >
> > >  static int virtnet_open(struct net_device *dev)
> > >  {
> > >       struct virtnet_info *vi = netdev_priv(dev);
> > > @@ -2903,6 +2923,16 @@ static int virtnet_open(struct net_device *dev)
> > >                       goto err_enable_qp;
> > >       }
> > >
> > > +     if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
> > > +             if (vi->status & VIRTIO_NET_S_LINK_UP)
> > > +                     netif_carrier_on(vi->dev);
> > > +             virtio_config_driver_enable(vi->vdev);
> > > +     } else {
> > > +             vi->status = VIRTIO_NET_S_LINK_UP;
> > > +             netif_carrier_on(dev);
> > > +             virtnet_update_settings(vi);
> > > +     }
> > > +
> > >       return 0;
> > >
> > >  err_enable_qp:
> > > @@ -3381,12 +3411,18 @@ static int virtnet_close(struct net_device *dev)
> > >       disable_delayed_refill(vi);
> > >       /* Make sure refill_work doesn't re-enable napi! */
> > >       cancel_delayed_work_sync(&vi->refill);
> > > +     /* Make sure config notification doesn't schedule config work */
> >
> > it's clear what this does even without a comment.
> > what you should comment on, and do not, is *why*.
> 
> Well, it just follows the existing style, for example the above said
> 
> "/* Make sure refill_work doesn't re-enable napi! */"

only at the grammar level.
you don't see the difference?

        /* Make sure refill_work doesn't re-enable napi! */
        cancel_delayed_work_sync(&vi->refill);

it explains why we cancel: to avoid re-enabling napi.

why do you cancel config callback and work?
comment should say that.

> >
> > > +     virtio_config_driver_disable(vi->vdev);
> > > +     /* Make sure status updating is cancelled */
> >
> > same
> >
> > also what "status updating"? confuses more than this clarifies.
> 
> Does "Make sure the config changed work is cancelled" sounds better?

no, this just repeats what code does.
explain why you cancel it.

-- 
MST