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

[Brian Kress]

    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)));