Re: [Qemu-devel] [PATCH] Fix incorrect small packet padding in vmxnet3

[Dmitry Fleytman]

> On Jun 28, 2015, at 18:34 PM, Brian Kress <address@hidden> wrote:
> 
>    When running ESXi under qemu there is an issue with the ESXi guest
> discarding packets that are too short.  The guest discards any packets
> under the normal minimum length for an ethernet packet (60).  This
> results in odd behaviour where other hosts or VMs on other hosts can
> communicate with the ESXi guest just fine (since there's a physical NIC
> somewhere doing padding), but VMs on the host and the host itself cannot
> because the ARP request packets are too small for the ESXi host to
> accept.
>    Someone in the past thought this was worth fixing, and added code to
> the vmxnet3 qemu emulation such that if it is receiving packets smaller
> than 60 bytes to pad the packet out to 60. Unfortunately this code is
> wrong (or at least in the wrong place). It does so BEFORE before taking
> into account the vnet_hdr at the front of the packet added by the tap
> device.  As a result, it might add padding, but it never adds enough.
>  Specifically it adds 10 less (the length of the vnet_hdr) than it
> needs to.
>    The following (hopefully "obviously correct") patch simply swaps
> the order of processing the vnet header and the padding.  With this
> patch an ESXi guest is able to communicate with the host or other local
> VMs.
> 
> Signed-off-by: Brian Kress <address@hidden>
> 
> --- a/hw/net/vmxnet3.c  2015-04-27 10:08:24.000000000 -0400
> +++ b/hw/net/vmxnet3.c  2015-06-23 11:38:48.865728713 -0400
> @@ -1879,6 +1879,12 @@
>         return -1;
>     }
> 
> +    if (s->peer_has_vhdr) {
> +        vmxnet_rx_pkt_set_vhdr(s->rx_pkt, (struct virtio_net_hdr *)buf);
> +        buf += sizeof(struct virtio_net_hdr);
> +        size -= sizeof(struct virtio_net_hdr);
> +    }
> +
>     /* Pad to minimum Ethernet frame length */
>     if (size < sizeof(min_buf)) {
>         memcpy(min_buf, buf, size);
> @@ -1887,12 +1893,6 @@
>         size = sizeof(min_buf);
>     }
> 
> -    if (s->peer_has_vhdr) {
> -        vmxnet_rx_pkt_set_vhdr(s->rx_pkt, (struct virtio_net_hdr *)buf);
> -        buf += sizeof(struct virtio_net_hdr);
> -        size -= sizeof(struct virtio_net_hdr);
> -    }
> -

Looks good.

~Dmitry.

>     vmxnet_rx_pkt_set_packet_type(s->rx_pkt,
>         get_eth_packet_type(PKT_GET_ETH_HDR(buf)));
>