[RFC PATCH 1/3] net: Pad short frames to minimum size (60 bytes)

[Bin Meng <bmeng.cn@gmail.com>]

From: Bin Meng <bin.meng@windriver.com>

The minimum Ethernet frame length is 60 bytes, and we should pad
frames whose length is smaller to the minimum size.

This commit fixes the issue as seen with various ethernet models,
that ARP requests get dropped, preventing the guest from becoming
visible on the network.

The following 2 commits that attempted to workaround this issue
in e1000 and vmxenet3 before, should be reverted.

  commit 78aeb23eded2 ("e1000: Pad short frames to minimum size (60 bytes)")
  commit 40a87c6c9b11 ("vmxnet3: Pad short frames to minimum size (60 bytes)")

Signed-off-by: Bin Meng <bin.meng@windriver.com>
---

 net/net.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/net.c b/net/net.c
index b038370..34004da 100644
--- a/net/net.c
+++ b/net/net.c
@@ -638,6 +638,7 @@ static ssize_t qemu_send_packet_async_with_flags(NetClientState *sender,
                                                  NetPacketSent *sent_cb)
 {
     NetQueue *queue;
+    uint8_t min_buf[60];
     int ret;
 
 #ifdef DEBUG_NET
@@ -649,6 +650,14 @@ static ssize_t qemu_send_packet_async_with_flags(NetClientState *sender,
         return size;
     }
 
+    /* Pad to minimum Ethernet frame length */
+    if (size < sizeof(min_buf)) {
+        memcpy(min_buf, buf, size);
+        memset(&min_buf[size], 0, sizeof(min_buf) - size);
+        buf = min_buf;
+        size = sizeof(min_buf);
+    }
+
     /* Let filters handle the packet first */
     ret = filter_receive(sender, NET_FILTER_DIRECTION_TX,
                          sender, flags, buf, size, sent_cb);
-- 
2.7.4