Re: [PATCH 3/3] vhost_net: fix virtio_net header endianness

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

On Fri, Feb 20, 2015 at 11:15:05AM +0100, Greg Kurz wrote:
> Without this patch, packets are being silently dropped by the tap backend.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

I think it's the wrong place to fix this.  You want a tun/macvtap ioctl
to enable legacy big endian stuff.  Treat it same way as vhost, with a
config option to enable.

> ---
>  drivers/vhost/net.c |   32 ++++++++++++++++++++++++++------
>  1 file changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index afa06d2..2923eee 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -288,6 +288,16 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>  	rcu_read_unlock_bh();
>  }
>  
> +static void fix_virtio_net_hdr(struct vhost_virtqueue *vq)
> +{
> +	struct virtio_net_hdr *hdr = vq->iov[0].iov_base;
> +
> +	hdr->hdr_len = vhost16_to_cpu(vq, hdr->hdr_len);
> +	hdr->gso_size = vhost16_to_cpu(vq, hdr->gso_size);
> +	hdr->csum_start  = vhost16_to_cpu(vq, hdr->csum_start);
> +	hdr->csum_offset = vhost16_to_cpu(vq, hdr->csum_offset);
> +}
> +
>  /* Expects to be always run from workqueue - which acts as
>   * read-size critical section for our kind of RCU. */
>  static void handle_tx(struct vhost_net *net)
> @@ -352,6 +362,10 @@ static void handle_tx(struct vhost_net *net)
>  			       "out %d, int %d\n", out, in);
>  			break;
>  		}
> +
> +		if (!hdr_size)
> +			fix_virtio_net_hdr(vq);
> +
>  		/* Skip header. TODO: support TSO. */
>  		len = iov_length(vq->iov, out);
>  		iov_iter_init(&msg.msg_iter, WRITE, vq->iov, out, len);
> @@ -609,12 +623,18 @@ static void handle_rx(struct vhost_net *net)
>  			continue;
>  		}
>  		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
> -		if (unlikely(vhost_hlen) &&
> -		    copy_to_iter(&hdr, sizeof(hdr), &fixup) != sizeof(hdr)) {
> -			vq_err(vq, "Unable to write vnet_hdr at addr %p\n",
> -			       vq->iov->iov_base);
> -			break;
> -		}
> +		if (unlikely(vhost_hlen)) {
> +			size_t len = copy_to_iter(&hdr, sizeof(hdr), &fixup);
> +
> +			if (len != sizeof(hdr)) {
> +				vq_err(vq,
> +				       "Unable to write vnet_hdr at addr %p\n",
> +				       vq->iov->iov_base);
> +				break;
> +			}
> +		} else
> +			fix_virtio_net_hdr(vq);
> +
>  		/* TODO: Should check and handle checksum. */
>  
>  		num_buffers = cpu_to_vhost16(vq, headcount);