Re: [dpdk-dev] [PATCH] net/virtio: fix mbuf data and pkt length mismatch

[Tiwei Bie <tiwei.bie@intel.com>]

On Mon, Oct 14, 2019 at 06:15:42PM +0300, Andrew Rybchenko wrote:
> > diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> > index 27ead19fb..822cce06d 100644
> > --- a/drivers/net/virtio/virtio_rxtx.c
> > +++ b/drivers/net/virtio/virtio_rxtx.c
> > @@ -597,9 +597,8 @@ virtqueue_enqueue_xmit_inorder(struct virtnet_tx *txvq,
> >   		dxp->cookie = (void *)cookies[i];
> >   		dxp->ndescs = 1;
> > -		hdr = (struct virtio_net_hdr *)
> > -			rte_pktmbuf_prepend(cookies[i], head_size);
> > -		cookies[i]->pkt_len -= head_size;
> > +		hdr = (struct virtio_net_hdr *)(char *)cookies[i]->buf_addr +
> > +			cookies[i]->data_off - head_size;
> >   		/* if offload disabled, hdr is not zeroed yet, do it now */
> >   		if (!vq->hw->has_tx_offload)
> > @@ -608,9 +607,10 @@ virtqueue_enqueue_xmit_inorder(struct virtnet_tx *txvq,
> >   			virtqueue_xmit_offload(hdr, cookies[i], true);
> >   		start_dp[idx].addr  = VIRTIO_MBUF_DATA_DMA_ADDR(cookies[i], vq);
> 
> As I understand the problem is here. It points to start of the packet
> (Ethernet header) since data_off is not changed above now, but
> should point to virtio_net_hdr before the packet.
> 
> I think the patch fixes the bug in a wrong direction. It looks better
> to simply remove
> 
>     cookies[i]->pkt_len -= head_size;
> 
> above and care about real packet length difference in
> virtio_update_packet_stats() or when it is called from Tx path.
> 
> If it is OK for maintainers I'm ready to send patches to rollback back
> this one and fix it as described above.

Hi Andrew,

Thanks for looking into this! Feel free to send your fix.
PS. Another thing also needs to be noticed is that, after
prepending the net hdr with rte_pktmbuf_prepend(), below
code in virtio_update_packet_stats() won't be able to
access the ether header as expected:

https://github.com/DPDK/dpdk/blob/31b798a6f08e9b333b94b8bb26910209aa810b73/drivers/net/virtio/virtio_rxtx.c#L134-L140

Thanks,
Tiwei