e1000: secrc support

Add support for secrc field. Reportedly needed by old RHEL guests.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/e1000.c

@@ -344,6 +344,15 @@ is_vlan_txd(uint32_t txd_lower)
     return ((txd_lower & E1000_TXD_CMD_VLE) != 0);
 }
 
+/* FCS aka Ethernet CRC-32. We don't get it from backends and can't
+ * fill it in, just pad descriptor length by 4 bytes unless guest
+ * told us to trip it off the packet. */
+static inline int
+fcs_len(E1000State *s)
+{
+    return (s->mac_reg[RCTL] & E1000_RCTL_SECRC) ? 0 : 4;
+}
+
 static void
 xmit_seg(E1000State *s)
 {
@@ -662,7 +671,7 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
         if (desc.buffer_addr) {
             cpu_physical_memory_write(le64_to_cpu(desc.buffer_addr),
                                       (void *)(buf + vlan_offset), size);
-            desc.length = cpu_to_le16(size + 4 /* for FCS */);
+            desc.length = cpu_to_le16(size + fcs_len(s));
             desc.status |= E1000_RXD_STAT_EOP|E1000_RXD_STAT_IXSM;
         } else // as per intel docs; skip descriptors with null buf addr
             DBGOUT(RX, "Null RX descriptor!!\n");