Re: [Qemu-devel] vmxnet3, vnet_hdr, and minimum length padding

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jun 23, 2015 at 11:49:25AM -0400, Brian Kress 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.
> 
> 
> --- a/qemu-2.3.0/hw/net/vmxnet3.c       2015-04-27 10:08:24.000000000 -0400
> +++ b/qemu-2.3.0/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);
> -    }
> -
>      vmxnet_rx_pkt_set_packet_type(s->rx_pkt,
>          get_eth_packet_type(PKT_GET_ETH_HDR(buf)));

Thanks, applied to my net tree:
https://github.com/stefanha/qemu/commits/net

Stefan

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