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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 19 Jun 2023 10:33:44 -0400

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 <[email-masked]>
> > > Reviewed-by: Xuan Zhuo <[email-masked]>
> > > ---
> > >  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?

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.

> > 
> > 
> > > @@ -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