virtio-net: add vnet_hdr support

With '-netdev tap,id=foo -nic model=virtio,netdev=foo' virtio-net can
detect that its peer (i.e. the tap backend) supports vnet headers
and advertise to the guest that it can send packets with partial
checksums and/or TSO packets.

One complication is that if we're migrating and the source host
supports IFF_VNET_HDR but the destination host doesn't, we can't then
stop the guest from using those features. In this scenario, we just
fail the migration.

[v2:
 - add has_vnet_hdr uint32_t field for ease of vmstate conversion
 - use qemu_error()
]
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-net.c

@@ -32,6 +32,7 @@ typedef struct VirtIONet
     VLANClientState *vc;
     QEMUTimer *tx_timer;
     int tx_timer_active;
+    uint32_t has_vnet_hdr;
     struct {
         VirtQueueElement elem;
         ssize_t len;
@@ -120,8 +121,22 @@ static void virtio_net_reset(VirtIODevice *vdev)
     memset(n->vlans, 0, MAX_VLAN >> 3);
 }
 
+static int peer_has_vnet_hdr(VirtIONet *n)
+{
+    if (!n->vc->peer)
+        return 0;
+
+    if (n->vc->peer->type != NET_CLIENT_TYPE_TAP)
+        return 0;
+
+    n->has_vnet_hdr = tap_has_vnet_hdr(n->vc->peer);
+
+    return n->has_vnet_hdr;
+}
+
 static uint32_t virtio_net_get_features(VirtIODevice *vdev)
 {
+    VirtIONet *n = to_virtio_net(vdev);
     uint32_t features = (1 << VIRTIO_NET_F_MAC) |
                         (1 << VIRTIO_NET_F_MRG_RXBUF) |
                         (1 << VIRTIO_NET_F_STATUS) |
@@ -130,6 +145,15 @@ static uint32_t virtio_net_get_features(VirtIODevice *vdev)
                         (1 << VIRTIO_NET_F_CTRL_VLAN) |
                         (1 << VIRTIO_NET_F_CTRL_RX_EXTRA);
 
+    if (peer_has_vnet_hdr(n)) {
+        tap_using_vnet_hdr(n->vc->peer, 1);
+
+        features |= (1 << VIRTIO_NET_F_CSUM);
+        features |= (1 << VIRTIO_NET_F_HOST_TSO4);
+        features |= (1 << VIRTIO_NET_F_HOST_TSO6);
+        features |= (1 << VIRTIO_NET_F_HOST_ECN);
+    }
+
     return features;
 }
 
@@ -359,6 +383,11 @@ static int receive_header(VirtIONet *n, struct iovec *iov, int iovcnt,
     hdr->flags = 0;
     hdr->gso_type = VIRTIO_NET_HDR_GSO_NONE;
 
+    if (n->has_vnet_hdr) {
+        memcpy(hdr, buf, sizeof(*hdr));
+        offset = sizeof(*hdr);
+    }
+
     /* We only ever receive a struct virtio_net_hdr from the tapfd,
      * but we may be passing along a larger header to the guest.
      */
@@ -378,6 +407,10 @@ static int receive_filter(VirtIONet *n, const uint8_t *buf, int size)
     if (n->promisc)
         return 1;
 
+    if (n->has_vnet_hdr) {
+        ptr += sizeof(struct virtio_net_hdr);
+    }
+
     if (!memcmp(&ptr[12], vlan, sizeof(vlan))) {
         int vid = be16_to_cpup((uint16_t *)(ptr + 14)) & 0xfff;
         if (!(n->vlans[vid >> 5] & (1U << (vid & 0x1f))))
@@ -510,7 +543,6 @@ static void virtio_net_tx_complete(VLANClientState *vc, ssize_t len)
 static void virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
 {
     VirtQueueElement elem;
-    int has_vnet_hdr = 0;
 
     if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
         return;
@@ -537,7 +569,7 @@ static void virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
         }
 
         /* ignore the header if GSO is not supported */
-        if (!has_vnet_hdr) {
+        if (!n->has_vnet_hdr) {
             out_num--;
             out_sg++;
             len += hdr_len;
@@ -610,7 +642,7 @@ static void virtio_net_save(QEMUFile *f, void *opaque)
     qemu_put_be32(f, n->mac_table.in_use);
     qemu_put_buffer(f, n->mac_table.macs, n->mac_table.in_use * ETH_ALEN);
     qemu_put_buffer(f, (uint8_t *)n->vlans, MAX_VLAN >> 3);
-    qemu_put_be32(f, 0); /* vnet-hdr placeholder */
+    qemu_put_be32(f, n->has_vnet_hdr);
     qemu_put_byte(f, n->mac_table.multi_overflow);
     qemu_put_byte(f, n->mac_table.uni_overflow);
     qemu_put_byte(f, n->alluni);
@@ -662,10 +694,15 @@ static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
     if (version_id >= 6)
         qemu_get_buffer(f, (uint8_t *)n->vlans, MAX_VLAN >> 3);
 
-    if (version_id >= 7 && qemu_get_be32(f)) {
-        fprintf(stderr,
-                "virtio-net: saved image requires vnet header support\n");
-        exit(1);
+    if (version_id >= 7) {
+        if (qemu_get_be32(f) && !peer_has_vnet_hdr(n)) {
+            qemu_error("virtio-net: saved image requires vnet_hdr=on\n");
+            return -1;
+        }
+
+        if (n->has_vnet_hdr) {
+            tap_using_vnet_hdr(n->vc->peer, 1);
+        }
     }
 
     if (version_id >= 9) {