e1000: multi-buffer packet support

e1000 supports multi-buffer packets larger than rxbuf_size.

This fixes the following (on linux):
- in guest: ifconfig eth1 mtu 16110
- in host: ifconfig tap0 mtu 16110
           ping -s 16082 <guest-ip>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: NAlex Williamson <alex.williamson@redhat.com>
Acked-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/e1000.c

@@ -642,6 +642,9 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
     uint16_t vlan_special = 0;
     uint8_t vlan_status = 0, vlan_offset = 0;
     uint8_t min_buf[MIN_BUF_SIZE];
+    size_t desc_offset;
+    size_t desc_size;
+    size_t total_size;
 
     if (!(s->mac_reg[RCTL] & E1000_RCTL_EN))
         return -1;
@@ -654,12 +657,6 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
         size = sizeof(min_buf);
     }
 
-    if (size > s->rxbuf_size) {
-        DBGOUT(RX, "packet too large for buffers (%lu > %d)\n",
-               (unsigned long)size, s->rxbuf_size);
-        return -1;
-    }
-
     if (!receive_filter(s, buf, size))
         return size;
 
@@ -672,8 +669,16 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
     }
 
     rdh_start = s->mac_reg[RDH];
+    desc_offset = 0;
+    total_size = size + fcs_len(s);
     do {
+        desc_size = total_size - desc_offset;
+        if (desc_size > s->rxbuf_size) {
+            desc_size = s->rxbuf_size;
+        }
         if (s->mac_reg[RDH] == s->mac_reg[RDT] && s->check_rxov) {
+            /* Discard all data written so far */
+            s->mac_reg[RDH] = rdh_start;
             set_ics(s, 0, E1000_ICS_RXO);
             return -1;
         }
@@ -683,10 +688,22 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
         desc.special = vlan_special;
         desc.status |= (vlan_status | E1000_RXD_STAT_DD);
         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 + fcs_len(s));
-            desc.status |= E1000_RXD_STAT_EOP|E1000_RXD_STAT_IXSM;
+            if (desc_offset < size) {
+                size_t copy_size = size - desc_offset;
+                if (copy_size > s->rxbuf_size) {
+                    copy_size = s->rxbuf_size;
+                }
+                cpu_physical_memory_write(le64_to_cpu(desc.buffer_addr),
+                                          (void *)(buf + desc_offset + vlan_offset),
+                                          copy_size);
+            }
+            desc_offset += desc_size;
+            if (desc_offset >= total_size) {
+                desc.length = cpu_to_le16(desc_size);
+                desc.status |= E1000_RXD_STAT_EOP | E1000_RXD_STAT_IXSM;
+            } else {
+                desc.length = cpu_to_le16(desc_size);
+            }
         } else { // as per intel docs; skip descriptors with null buf addr
             DBGOUT(RX, "Null RX descriptor!!\n");
         }
@@ -702,7 +719,7 @@ e1000_receive(VLANClientState *nc, const uint8_t *buf, size_t size)
             set_ics(s, 0, E1000_ICS_RXO);
             return -1;
         }
-    } while (desc.buffer_addr == 0);
+    } while (desc_offset < total_size);
 
     s->mac_reg[GPRC]++;
     s->mac_reg[TPR]++;