Re: [PATCH net-next 4/4] virtio-net: remove F_GUEST_CSUM check for XDP loading

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 19, 2023 at 11:43:29PM +0800, Heng Qi wrote:
> On Mon, Jun 19, 2023 at 10:33:44AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Jun 19, 2023 at 08:41:54PM +0800, Heng Qi wrote:
> > > On Mon, Jun 19, 2023 at 07:16:20AM -0400, Michael S. Tsirkin wrote:
> > > > On Mon, Jun 19, 2023 at 06:57:38PM +0800, Heng Qi wrote:
> > > > > Lay the foundation for the subsequent patch
> > > > 
> > > > which subsequent patch? this is the last one in series.
> > > > 
> > > > > to complete the coexistence
> > > > > of XDP and virtio-net guest csum.
> > > > > 
> > > > > Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> > > > > Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > > ---
> > > > >  drivers/net/virtio_net.c | 4 +---
> > > > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 25b486ab74db..79471de64b56 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -60,7 +60,6 @@ static const unsigned long guest_offloads[] = {
> > > > >  	VIRTIO_NET_F_GUEST_TSO6,
> > > > >  	VIRTIO_NET_F_GUEST_ECN,
> > > > >  	VIRTIO_NET_F_GUEST_UFO,
> > > > > -	VIRTIO_NET_F_GUEST_CSUM,
> > > > >  	VIRTIO_NET_F_GUEST_USO4,
> > > > >  	VIRTIO_NET_F_GUEST_USO6,
> > > > >  	VIRTIO_NET_F_GUEST_HDRLEN
> > > > 
> > > > What is this doing? Drop support for VIRTIO_NET_F_GUEST_CSUM? Why?
> > > 
> > > guest_offloads[] is used by the VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET
> > > command to switch features when XDP is loaded/unloaded.
> > > 
> > > If the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature is negotiated:
> > > 1. When XDP is loaded, virtnet_xdp_set() uses virtnet_clear_guest_offloads()
> > > to automatically turn off the features in guest_offloads[].
> > > 
> > > 2. when XDP is unloaded, virtnet_xdp_set() uses virtnet_restore_guest_offloads()
> > > to automatically restore the features in guest_offloads[].
> > > 
> > > Now, this work no longer makes XDP and _F_GUEST_CSUM mutually
> > > exclusive, so this patch removed the _F_GUEST_CSUM from guest_offloads[].
> > > 
> > > > This will disable all of guest offloads I think ..
> > > 
> > > No. This doesn't change the dependencies of other features on
> > > _F_GUEST_CSUM. Removing _F_GUEST_CSUM here does not mean that other
> > > features that depend on it will be turned off at the same time, such as
> > > _F_GUEST_TSO{4,6}, F_GUEST_USO{4,6}, etc.
> > > 
> > > Thanks.
> > 
> > Hmm I don't get it.
> > 
> > static int virtnet_restore_guest_offloads(struct virtnet_info *vi)
> > {               
> >         u64 offloads = vi->guest_offloads;
> >                         
> >         if (!vi->guest_offloads)
> >                 return 0;
> >         
> >         return virtnet_set_guest_offloads(vi, offloads); 
> > }               
> >                         
> > is the bit _F_GUEST_CSUM set in vi->guest_offloads?
> 
> No, but first we doesn't clear _F_GUEST_CSUM in virtnet_clear_guest_offloads().
> 
> If VIRTIO_NET_F_CTRL_GUEST_OFFLOADS is negotiated, features that can
> be dynamically controlled by the VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET
> command must also be negotiated. Therefore, if GRO_HW_MASK features such
> as _F_GUEST_TSO exist, then _F_GUEST_CSUM must exist (according to the
> dependencies defined by the spec).
> 
> Now, we only dynamically turn off/on the features contained in
> guest_offloads[] through XDP loading/unloading (with this patch,
> _F_GUEST_CSUM will not be controlled), and _F_GUEST_CSUM is always on.
> 
> Another point is that the virtio-net NETIF_F_RXCSUM corresponding to
> _F_GUEST_CSUM is only included in dev->features, not in dev->hw_features,
> which means that users cannot manually control the switch of
> _F_GUEST_CSUM.

I think I get it now.

> > 
> > Because if it isn't then we'll try to set _F_GUEST_TSO
> > without setting _F_GUEST_CSUM and that's a spec
> > violation I think.
> 
> As explained above, we did not cause a specification violation.
> 
> Thanks.

Right.

> > 
> > 
> > > > 
> > > > 
> > > > > @@ -3522,10 +3521,9 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> > > > >  	        virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_TSO6) ||
> > > > >  	        virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_ECN) ||
> > > > >  		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_UFO) ||
> > > > > -		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_CSUM) ||
> > > > >  		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_USO4) ||
> > > > >  		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_USO6))) {
> > > > > -		NL_SET_ERR_MSG_MOD(extack, "Can't set XDP while host is implementing GRO_HW/CSUM, disable GRO_HW/CSUM first");
> > > > > +		NL_SET_ERR_MSG_MOD(extack, "Can't set XDP while host is implementing GRO_HW, disable GRO_HW first");
> > > > >  		return -EOPNOTSUPP;
> > > > >  	}
> > > > >  
> > > > > -- 
> > > > > 2.19.1.6.gb485710b