Subject: Re: [PATCH net-next 3/4] virtio-net: support coexistence of XDP and _F_GUEST_CSUM

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 20 Jun 2023 06:50:34 -0400

On Tue, Jun 20, 2023 at 11:24:30AM +0800, Heng Qi wrote:
> On Mon, Jun 19, 2023 at 07:26:44AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Jun 19, 2023 at 06:57:37PM +0800, Heng Qi wrote:
> > > We are now re-probing the csum related fields and  trying
> > > to have XDP and RX hw checksum capabilities coexist on the
> > > XDP path. For the benefit of:
> > > 1. RX hw checksum capability can be used if XDP is loaded.
> > > 2. Avoid packet loss when loading XDP in the vm-vm scenario.
> > > 
> > > Signed-off-by: Heng Qi <[email-masked]>
> > > Reviewed-by: Xuan Zhuo <[email-masked]>
> > > ---
> > >  drivers/net/virtio_net.c | 36 ++++++++++++++++++++++++------------
> > >  1 file changed, 24 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 07b4801d689c..25b486ab74db 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -1709,6 +1709,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > >  	struct net_device *dev = vi->dev;
> > >  	struct sk_buff *skb;
> > >  	struct virtio_net_hdr_mrg_rxbuf *hdr;
> > > +	__u8 flags;
> > >  
> > >  	if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
> > >  		pr_debug("%s: short packet %i\n", dev->name, len);
> > > @@ -1717,6 +1718,8 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > >  		return;
> > >  	}
> > >  
> > > +	flags = ((struct virtio_net_hdr_mrg_rxbuf *)buf)->hdr.flags;
> > > +
> > >  	if (vi->mergeable_rx_bufs)
> > >  		skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
> > >  					stats);
> > 
> > what's going on here?
> 
> Hi, Michael.
> 
> Is your question about the function of this code?
> 1. If yes,
> this sentence saves the flags value in virtio-net-hdr in advance
> before entering the XDP processing logic, so that it can be used to
> judge further logic after XDP processing.
> 
> If _NEEDS_CSUM is included in flags before XDP processing, then after
> XDP processing we need to re-probe the csum fields and calculate the
> pseudo-header checksum.

Yes but we previously used this:
-       hdr = skb_vnet_hdr(skb);
which pokes at the copy in skb cb.

Is anything wrong with this?

It seems preferable not to poke at the header an extra time.

-- 
MST