Re: [PATCH net-next V2] virtio_net: ethtool tx napi configuration

Implement ethtool .set_coalesce (-C) and .get_coalesce (-c) handlers.
Interrupt moderation is currently not supported, so these accept and
display the default settings of 0 usec and 1 frame.

Toggle tx napi through a bit in tx-frames. So as to not interfere
with possible future interrupt moderation, value 1 means tx napi while
value 0 means not.

To properly synchronize with the data path, tx napi is disabled and
tx lock is held when changing the value of napi weight. And two more
places that can access tx napi weight:

- speculative tx polling in rx napi, we can leave it as is since it
  not a must for correctness.
- skb_xmit_done(), one more check of napi weight is added before
  trying to enable tx to avoid tx to be disabled forever if napi is
  disabled after skb_xmit_done() but before the napi

Link: https://patchwork.ozlabs.org/patch/948149/
Suggested-by: Jason Wang <redacted>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jason Wang <redacted>
---
Changes from V1:
- try to synchronize with datapath to allow changing mode when
  interface is up.
- use tx-frames 0 as to disable tx napi while tx-frames 1 to enable tx napi
---
 drivers/net/virtio_net.c | 64 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 765920905226..6e70864f5899 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -66,6 +66,8 @@ DECLARE_EWMA(pkt_len, 0, 64)

 #define VIRTNET_DRIVER_VERSION "1.0.0"

+static const u32 ethtool_coalesce_napi_mask = (1UL << 10);
+

 static const unsigned long guest_offloads[] = {
        VIRTIO_NET_F_GUEST_TSO4,
        VIRTIO_NET_F_GUEST_TSO6,

@@ -1444,7 +1446,10 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)

        virtqueue_napi_complete(napi, sq->vq, 0);

-       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
+       /* Check napi.weight to avoid tx stall since it could be set
+        * to zero by ethtool after skb_xmit_done().
+        */
+       if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS || !sq->napi.weight)
                netif_tx_wake_queue(txq);

        return 0;

@@ -2181,6 +2186,61 @@ static int virtnet_get_link_ksettings(struct net_device *dev,
        return 0;
 }

+static int virtnet_set_coalesce(struct net_device *dev,
+                               struct ethtool_coalesce *ec)
+{
+       struct ethtool_coalesce ec_default = {
+               .cmd = ETHTOOL_SCOALESCE,
+               .rx_max_coalesced_frames = 1,
+       };
+       struct virtnet_info *vi = netdev_priv(dev);
+       int i, napi_weight;
+
+       if (ec->tx_max_coalesced_frames > 1)
+               return -EINVAL;
+
+       ec_default.tx_max_coalesced_frames = ec->tx_max_coalesced_frames;
+       napi_weight = ec->tx_max_coalesced_frames ? NAPI_POLL_WEIGHT : 0;
+
+       /* disallow changes to fields not explicitly tested above */
+       if (memcmp(ec, &ec_default, sizeof(ec_default)))
+               return -EINVAL;
+
+       if (napi_weight ^ vi->sq[0].napi.weight) {
+               for (i = 0; i < vi->max_queue_pairs; i++) {
+                       struct netdev_queue *txq =
+                              netdev_get_tx_queue(vi->dev, i);
+
+                       virtnet_napi_tx_disable(&vi->sq[i].napi);
+                       __netif_tx_lock_bh(txq);
+                       vi->sq[i].napi.weight = napi_weight;
+                       __netif_tx_unlock_bh(txq);
+                       virtnet_napi_tx_enable(vi, vi->sq[i].vq,
+                                              &vi->sq[i].napi);
+               }
+       }
+
+       return 0;
+}
+
+static int virtnet_get_coalesce(struct net_device *dev,
+                               struct ethtool_coalesce *ec)
+{
+       struct ethtool_coalesce ec_default = {
+               .cmd = ETHTOOL_GCOALESCE,
+               .rx_max_coalesced_frames = 1,
+               .tx_max_coalesced_frames = 0,

+       };
+       struct virtnet_info *vi = netdev_priv(dev);
+
+       memcpy(ec, &ec_default, sizeof(ec_default));
+
+       if (vi->sq[0].napi.weight)
+               ec->tx_max_coalesced_frames = 1;
+
+       return 0;
+}
+
 static void virtnet_init_settings(struct net_device *dev)
 {
        struct virtnet_info *vi = netdev_priv(dev);

@@ -2219,6 +2279,8 @@ static const struct ethtool_ops virtnet_ethtool_ops = {
        .get_ts_info = ethtool_op_get_ts_info,
        .get_link_ksettings = virtnet_get_link_ksettings,
        .set_link_ksettings = virtnet_set_link_ksettings,
+       .set_coalesce = virtnet_set_coalesce,
+       .get_coalesce = virtnet_get_coalesce,
 };

 static void virtnet_freeze_down(struct virtio_device *vdev)
--

2.17.1