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 08:10:38 -0400

On Tue, Jun 20, 2023 at 07:01:48PM +0800, Heng Qi wrote:
> On Tue, Jun 20, 2023 at 06:50:34AM -0400, Michael S. Tsirkin wrote:
> > 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?
> > 
> 
> This is where we save the hdr when there is no XDP loaded (note that
> this is the complete hdr, including flags, and also including GSO and
> other information). When XDP is loaded, because hdr is invalid, we will
> not save it into skb->cb.
> 
> But the above situation is not what we want. Now our purpose is to save
> the hdr information before XDP processing, that is, when the driver has
> just received the packet and has not built the skb (in fact, we only
> need flags). Therefore, only flags are saved here.
> 
> Thanks.

I don't get it. this seems to be the only use of flags:

+       if (unlikely(vi->xdp_enabled)) {
+               if (virtnet_set_csum_after_xdp(vi, skb, flags) < 0) {
+                       pr_debug("%s: errors occurred in flow dissector setting csum",
+                                dev->name);
+                       goto frame_err;
+               }

looks like skb has already been created here.

is there another use of flags that I missed?

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