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

[Koushik Dutta <kdutta@redhat.com>]

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Hi Eugenio,
Regarding the point about not enabling VIRTIO_NET_F_NOTF_COAL
unconditionally: since feature negotiation happens during VM bootup and
coalescing is configured via ethtool after the VM has already booted, how
would we handle this under a conditional property?

If the feature isn't negotiated at the start, we wouldn't be able to enable
it dynamically later. Could you clarify how you'd like the command-line
regulation to interact with the guest's ability to enable it later?
------------------------------------------------------------

> @@ -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:

This patch does not handle VIRTIO_F_RING_RESET, so this should not be
enabled.

=> I included VIRTIO_F_RING_RESET in the valid features list because the
vDPA host device offers it by default; without it, SVQ fails the feature
negotiation check during initialization.
qemu-system-x86_64: -netdev
type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa0,x-svq=on: SVQ
Invalid device feature flags, offer: 0x1e0010330bfffa7, ok:
0x1e0000330bfffa7

Regards,
Koushik Dutta

On Thu, Mar 12, 2026 at 3:47 PM Eugenio Perez Martin <eperezma@redhat.com>
wrote:

> On Wed, Mar 11, 2026 at 2:28 PM Koushik Dutta <kdutta@redhat.com> wrote:
> >
> > 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) {
>
> Maybe it's better to check if !timer_pending? It wasn't immediately
> clear to me that `n->rx_pkt_cnt == i` until I realized that
> `n->rx_pkt_cnt` is 0 at the beginning of the function for that
> condition to be true.
>
> > +            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;
>
> If we got his way, we must control for underflow here.
>
> But I think it should be n->pt_pkt_cnt = 0, isn't it?
>
> >          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;
>
> No declarations in the middle of the function.
>
> Also, why use_timer = true as long as 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]);
>
> We should avoid creating a new timer if it will not be used by
> default. Create it only if cmdline enables it or the guest enable it
> via control vq.
>
> > +
> > +    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);
>
> We shouldn't enable VIRTIO_NET_F_NOTF_COAL unconditionally, it should
> be regulated by the cmdline property.
>
> >
> >      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);
>
> Same as the tx timer. Is it worth creating a new timer even if it will
> not be used by default?
>
> >  }
> >
> >  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:
>
> This patch does not handle VIRTIO_F_RING_RESET, so this should not be
> enabled.
>
> >          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,
>
> Only this feature can be added to vdpa_feature_bits. The rest of them
> must not be added to any other array.
>
> >      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
> >
>
>

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