[PATCH v2] hw/net/virtio-net: add support for notification coalescing

[Koushik Dutta <kdutta@redhat.com>]

Implement VirtIO Network Notification Coalescing (Bit 53).
This allows the guest to manage interrupt frequency using ethtool
-C for both RX and TX paths.

- Added VIRTIO_NET_F_NOTF_COAL to host features.
- Implemented VIRTIO_NET_CTRL_NOTF_COAL class handling in
  virtio_net_handle_ctrl_iov.
- Added logic to store and apply rx/tx usecs and max_packets.
- Added packet counters and threshold logic for both RX and TX data paths.
- Dynamic Dispatcher: Implemented a dispatcher mechanism that
  dynamically switches/activates the notification callback logic
  only after the guest enables TX coalescing via ethtool.

This reduces interrupt overhead by batching notifications based on
either a packet count or a time-based threshold.

Signed-off-by: Koushik Dutta <kdutta@redhat.com>
---
 hw/net/virtio-net.c                | 114 ++++++++++++++++++++++++-----
 hw/virtio/vhost-shadow-virtqueue.c |   1 +
 hw/virtio/vhost-vdpa.c             |   3 +
 include/hw/virtio/virtio-net.h     |   8 ++
 net/vhost-vdpa.c                   |  10 +++
 5 files changed, 119 insertions(+), 17 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index eccb48ad42..96aa70aea8 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -157,6 +157,15 @@ static void flush_or_purge_queued_packets(NetClientState *nc)
  * - we could suppress RX interrupt if we were so inclined.
  */
 
+static void virtio_net_rx_notify(void *opaque)
+{
+    VirtIONet *n = opaque;
+    VirtIODevice *vdev = VIRTIO_DEVICE(n);
+
+    n->rx_pkt_cnt = 0;
+    virtio_notify(vdev, n->vqs[0].rx_vq);
+}
+
 static void virtio_net_get_config(VirtIODevice *vdev, uint8_t *config)
 {
     VirtIONet *n = VIRTIO_NET(vdev);
@@ -1081,6 +1090,29 @@ static int virtio_net_handle_offloads(VirtIONet *n, uint8_t cmd,
     }
 }
 
+static int virtio_net_handle_coal(VirtIONet *n, uint8_t cmd,
+                                  struct iovec *iov, unsigned int iov_cnt)
+{
+    struct virtio_net_ctrl_coal coal;
+    size_t s;
+
+    s = iov_to_buf(iov, iov_cnt, 0, &coal, sizeof(coal));
+    if (s != sizeof(coal)) {
+        return VIRTIO_NET_ERR;
+    }
+
+    if (cmd == VIRTIO_NET_CTRL_NOTF_COAL_RX_SET) {
+        n->rx_coal_usecs = le32_to_cpu(coal.max_usecs);
+        n->rx_coal_packets = le32_to_cpu(coal.max_packets);
+    } else if (cmd == VIRTIO_NET_CTRL_NOTF_COAL_TX_SET) {
+        n->tx_coal_usecs = le32_to_cpu(coal.max_usecs);
+        n->tx_coal_packets = le32_to_cpu(coal.max_packets);
+        n->tx_timeout = n->tx_coal_usecs * 1000;
+    }
+
+    return VIRTIO_NET_OK;
+}
+
 static int virtio_net_handle_mac(VirtIONet *n, uint8_t cmd,
                                  struct iovec *iov, unsigned int iov_cnt)
 {
@@ -1582,6 +1614,8 @@ size_t virtio_net_handle_ctrl_iov(VirtIODevice *vdev,
         status = virtio_net_handle_mq(n, ctrl.cmd, iov, out_num);
     } else if (ctrl.class == VIRTIO_NET_CTRL_GUEST_OFFLOADS) {
         status = virtio_net_handle_offloads(n, ctrl.cmd, iov, out_num);
+    } else if (ctrl.class == VIRTIO_NET_CTRL_NOTF_COAL) {
+        status = virtio_net_handle_coal(n, ctrl.cmd, iov, out_num);
     }
 
     s = iov_from_buf(in_sg, in_num, 0, &status, sizeof(status));
@@ -2041,7 +2075,18 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
     }
 
     virtqueue_flush(q->rx_vq, i);
-    virtio_notify(vdev, q->rx_vq);
+
+    /* rx coalescing */
+    n->rx_pkt_cnt += i;
+    if (n->rx_coal_usecs == 0 || n->rx_pkt_cnt >= n->rx_coal_packets) {
+        timer_del(n->rx_index_timer);
+        virtio_net_rx_notify(n);
+    } else {
+        if (n->rx_pkt_cnt == i) {
+            timer_mod(n->rx_index_timer,
+                      qemu_clock_get_us(QEMU_CLOCK_VIRTUAL) + n->rx_coal_usecs);
+        }
+    }
 
     return size;
 
@@ -2900,6 +2945,7 @@ static void virtio_net_tx_timer(void *opaque)
     if (ret == -EBUSY || ret == -EINVAL) {
         return;
     }
+    n->tx_pkt_cnt -= ret;
     /*
      * If we flush a full burst of packets, assume there are
      * more coming and immediately rearm
@@ -2919,6 +2965,7 @@ static void virtio_net_tx_timer(void *opaque)
     ret = virtio_net_flush_tx(q);
     if (ret > 0) {
         virtio_queue_set_notification(q->tx_vq, 0);
+        n->tx_pkt_cnt -= ret;
         q->tx_waiting = 1;
         timer_mod(q->tx_timer,
                   qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + n->tx_timeout);
@@ -2974,27 +3021,45 @@ static void virtio_net_tx_bh(void *opaque)
     }
 }
 
-static void virtio_net_add_queue(VirtIONet *n, int index)
+static void virtio_net_handle_tx_dispatch(VirtIODevice *vdev, VirtQueue *vq)
 {
-    VirtIODevice *vdev = VIRTIO_DEVICE(n);
+    VirtIONet *n = VIRTIO_NET(vdev);
 
-    n->vqs[index].rx_vq = virtio_add_queue(vdev, n->net_conf.rx_queue_size,
-                                           virtio_net_handle_rx);
+    n->tx_pkt_cnt++;
+    bool use_timer = n->tx_timer_activate || n->tx_coal_usecs > 0 ||
+                     n->tx_coal_packets > 0;
 
-    if (n->net_conf.tx && !strcmp(n->net_conf.tx, "timer")) {
-        n->vqs[index].tx_vq =
-            virtio_add_queue(vdev, n->net_conf.tx_queue_size,
-                             virtio_net_handle_tx_timer);
-        n->vqs[index].tx_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
-                                              virtio_net_tx_timer,
-                                              &n->vqs[index]);
+    if (use_timer && n->tx_pkt_cnt < n->tx_coal_packets) {
+        virtio_net_handle_tx_timer(vdev, vq);
     } else {
-        n->vqs[index].tx_vq =
-            virtio_add_queue(vdev, n->net_conf.tx_queue_size,
-                             virtio_net_handle_tx_bh);
-        n->vqs[index].tx_bh = qemu_bh_new_guarded(virtio_net_tx_bh, &n->vqs[index],
-                                                  &DEVICE(vdev)->mem_reentrancy_guard);
+        n->tx_pkt_cnt = 0;
+        virtio_net_handle_tx_bh(vdev, vq);
     }
+}
+
+static void virtio_net_add_queue(VirtIONet *n, int index)
+{
+    VirtIODevice *vdev = VIRTIO_DEVICE(n);
+
+    n->vqs[index].rx_vq =
+        virtio_add_queue(vdev,
+                         n->net_conf.rx_queue_size,
+                         virtio_net_handle_rx);
+
+    n->vqs[index].tx_vq =
+        virtio_add_queue(vdev,
+                         n->net_conf.tx_queue_size,
+                         virtio_net_handle_tx_dispatch);
+
+    n->vqs[index].tx_timer =
+        timer_new_ns(QEMU_CLOCK_VIRTUAL,
+                     virtio_net_tx_timer,
+                     &n->vqs[index]);
+
+    n->vqs[index].tx_bh =
+        qemu_bh_new_guarded(virtio_net_tx_bh,
+                            &n->vqs[index],
+                            &DEVICE(vdev)->mem_reentrancy_guard);
 
     n->vqs[index].tx_waiting = 0;
     n->vqs[index].n = n;
@@ -3089,6 +3154,7 @@ static void virtio_net_get_features(VirtIODevice *vdev, uint64_t *features,
     virtio_features_or(features, features, n->host_features_ex);
 
     virtio_add_feature_ex(features, VIRTIO_NET_F_MAC);
+    virtio_add_feature_ex(features, VIRTIO_NET_F_NOTF_COAL);
 
     if (!peer_has_vnet_hdr(n)) {
         virtio_clear_feature_ex(features, VIRTIO_NET_F_CSUM);
@@ -3972,6 +4038,10 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
         error_printf("Defaulting to \"bh\"");
     }
 
+    if (n->net_conf.tx && strcmp(n->net_conf.tx, "timer")) {
+        n->tx_timer_activate = true;
+    }
+
     n->net_conf.tx_queue_size = MIN(virtio_net_max_tx_queue_size(n),
                                     n->net_conf.tx_queue_size);
 
@@ -4048,6 +4118,14 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
             n->rss_data.specified_hash_types.on_bits |
             n->rss_data.specified_hash_types.auto_bits;
     }
+    n->rx_pkt_cnt = 0;
+    n->tx_pkt_cnt = 0;
+    n->rx_coal_usecs = 0;
+    n->tx_coal_usecs = 0;
+    n->rx_coal_packets = 0;
+    n->tx_coal_packets = 0;
+    n->rx_index_timer = timer_new_us(QEMU_CLOCK_VIRTUAL,
+                                     virtio_net_rx_notify, n);
 }
 
 static void virtio_net_device_unrealize(DeviceState *dev)
@@ -4262,6 +4340,8 @@ static const Property virtio_net_properties[] = {
                       VIRTIO_NET_F_GUEST_USO6, true),
     DEFINE_PROP_BIT64("host_uso", VirtIONet, host_features,
                       VIRTIO_NET_F_HOST_USO, true),
+    DEFINE_PROP_BIT64("vq_notf_coal", VirtIONet, host_features,
+                      VIRTIO_NET_F_NOTF_COAL, true),
     DEFINE_PROP_ON_OFF_AUTO_BIT64("hash-ipv4", VirtIONet,
                                   rss_data.specified_hash_types,
                                   VIRTIO_NET_HASH_REPORT_IPv4 - 1,
diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 6242aeb69c..104ed0ce8f 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -37,6 +37,7 @@ bool vhost_svq_valid_features(uint64_t features, Error **errp)
         case VIRTIO_RING_F_INDIRECT_DESC:
             continue;
 
+        case VIRTIO_F_RING_RESET:
         case VIRTIO_F_ACCESS_PLATFORM:
             /* SVQ trust in the host's IOMMU to translate addresses */
         case VIRTIO_F_VERSION_1:
diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 2f8f11df86..8eb9bdb961 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -1541,6 +1541,9 @@ static int vhost_vdpa_get_features(struct vhost_dev *dev,
     if (ret == 0) {
         /* Add SVQ logging capabilities */
         *features |= BIT_ULL(VHOST_F_LOG_ALL);
+
+        /* Add notification coalescing features */
+        *features |= BIT_ULL(VIRTIO_NET_F_NOTF_COAL);
     }
 
     return ret;
diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
index 5b8ab7bda7..024501ed37 100644
--- a/include/hw/virtio/virtio-net.h
+++ b/include/hw/virtio/virtio-net.h
@@ -231,6 +231,14 @@ struct VirtIONet {
     struct EBPFRSSContext ebpf_rss;
     uint32_t nr_ebpf_rss_fds;
     char **ebpf_rss_fds;
+    QEMUTimer *rx_index_timer;
+    uint32_t rx_coal_usecs;
+    uint32_t rx_coal_packets;
+    uint32_t rx_pkt_cnt;
+    uint32_t tx_coal_usecs;
+    uint32_t tx_coal_packets;
+    uint32_t tx_pkt_cnt;
+    bool tx_timer_activate;
 };
 
 size_t virtio_net_handle_ctrl_iov(VirtIODevice *vdev,
diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
index 3df6091274..6a5d0f4cae 100644
--- a/net/vhost-vdpa.c
+++ b/net/vhost-vdpa.c
@@ -61,6 +61,7 @@ static const int vdpa_feature_bits[] = {
     VIRTIO_F_NOTIFY_ON_EMPTY,
     VIRTIO_F_RING_PACKED,
     VIRTIO_F_RING_RESET,
+    VIRTIO_F_ORDER_PLATFORM,
     VIRTIO_F_VERSION_1,
     VIRTIO_F_IN_ORDER,
     VIRTIO_F_NOTIFICATION_DATA,
@@ -70,6 +71,8 @@ static const int vdpa_feature_bits[] = {
     VIRTIO_NET_F_CTRL_RX,
     VIRTIO_NET_F_CTRL_RX_EXTRA,
     VIRTIO_NET_F_CTRL_VLAN,
+    VIRTIO_NET_F_GUEST_ANNOUNCE,
+    VIRTIO_NET_F_NOTF_COAL,
     VIRTIO_NET_F_CTRL_VQ,
     VIRTIO_NET_F_GSO,
     VIRTIO_NET_F_GUEST_CSUM,
@@ -115,6 +118,13 @@ static const uint64_t vdpa_svq_device_features =
     BIT_ULL(VIRTIO_NET_F_HOST_UFO) |
     BIT_ULL(VIRTIO_NET_F_MRG_RXBUF) |
     BIT_ULL(VIRTIO_NET_F_STATUS) |
+    BIT_ULL(VIRTIO_F_RING_RESET) |
+    BIT_ULL(VIRTIO_F_ORDER_PLATFORM) |
+    BIT_ULL(VIRTIO_NET_F_GUEST_USO4) |
+    BIT_ULL(VIRTIO_NET_F_GUEST_USO6) |
+    BIT_ULL(VIRTIO_NET_F_HOST_USO) |
+    BIT_ULL(VIRTIO_NET_F_GUEST_ANNOUNCE) |
+    BIT_ULL(VIRTIO_NET_F_NOTF_COAL) |
     BIT_ULL(VIRTIO_NET_F_CTRL_VQ) |
     BIT_ULL(VIRTIO_NET_F_GSO) |
     BIT_ULL(VIRTIO_NET_F_CTRL_RX) |
-- 
2.53.0