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

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/06/2015 15:34, Brian Kress wrote:
> Resending by request with Signed-off-by:
> 
> 
>     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.

This is not the correct format for a patch:

- the subject should start with [PATCH]

- the subject should describe what the patch does

- the lines of the body of the commit message should be ~72 characters
long at most

- the patch should apply with "patch -p1" (your patch requires "-p2").

For more information, see http://wiki.qemu.org/Contribute/SubmitAPatch.

It's up to the maintainer whether to fix the above; the code however is
fine so:

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Thanks,

Paolo


> 
> Signed-off-by: Brian Kress <kressb@moose.net>
> 
> --- 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)));
> 
> 
>