On Wed, Sep 16, 2026 at 7:51 PM Eugenio Perez Martin <[email protected]> wrote:
> On Thu, Jul 16, 2026 at 3:44 PM Koushik Dutta <[email protected]> wrote: > > > > The feature supports two coalescing modes: > > - Time-based: delay notifications up to N microseconds > > - Count-based: delay until N packets are processed > > > > Implementation details: > > - Added VIRTIO_NET_CTRL_NOTF_COAL class handling in control virtqueue > > - RX path: batches notifications based on packet count or timeout > > - TX path: leverages the unified dispatcher to dynamically enable > > timer-based coalescing when guest configures it via ethtool > > - Coalescing parameters persist across live migration > > > > Note: When tx=timer is configured at VM launch, the coalescing feature > > is automatically disabled because tx=timer already introduces a fixed > > 150µs delay for packet batching, making notification coalescing > redundant. > > > > Where does this 150µs value come from? The actual value is specified > in the cmdline in ns. > > And that's not what I meant back then. I think the best action is to > forbid the guest from changing the coalescing settings because it's > hard to determine which setting should take precedence, even if the > VirtIO specifies it is best effort. Let me know if I misunderstood > your sentence so we can make it more accurate. > > > Adds the `x-vq_notf_coal=on` device parameter to dynamically enable > > virtqueue notification coalescing at VM startup. > > > > Performance metrics (Moderate Coalescing vs Without Coalescing): > > - RX parameters: 1000µs, 10 frames > > - TX parameters: 2000µs, 20 frames > > - Results: 20% drop in CPU overhead and a 30% reduction in interrupts. > > > > Signed-off-by: Koushik Dutta <[email protected]> > > --- > > hw/net/virtio-net.c | 203 ++++++++++++++++++++++++++++++++- > > include/hw/virtio/virtio-net.h | 10 ++ > > net/passt.c | 1 + > > net/tap.c | 1 + > > net/vhost-user.c | 1 + > > net/vhost-vdpa.c | 1 + > > 6 files changed, 213 insertions(+), 4 deletions(-) > > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > > index e7f05956a5..fac1013d47 100644 > > --- a/hw/net/virtio-net.c > > +++ b/hw/net/virtio-net.c > > @@ -157,6 +157,16 @@ 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) > > +{ > > + VirtIONetQueue *q = opaque; > > + VirtIONet *n = q->n; > > + VirtIODevice *vdev = VIRTIO_DEVICE(n); > > + > > + q->rx_coal.pkt_cnt = 0; > > + virtio_notify(vdev, q->rx_vq); > > +} > > + > > static void virtio_net_get_config(VirtIODevice *vdev, uint8_t *config) > > { > > VirtIONet *n = VIRTIO_NET(vdev); > > @@ -1002,6 +1012,87 @@ static void virtio_net_set_features(VirtIODevice > *vdev, > > } > > } > > > > +static void virtio_net_tx_notification_timer(void *opaque) > > +{ > > + VirtIONetQueue *q = opaque; > > + VirtIONet *n = q->n; > > + VirtIODevice *vdev = VIRTIO_DEVICE(n); > > + > > + q->tx_coal.pkt_cnt = 0; > > + virtio_notify(vdev, q->tx_vq); > > +} > > + > > +static bool virtio_net_tx_should_notify(VirtIONetQueue *q) > > +{ > > + if (q->tx_coal.usecs == 0 || !q->tx_coal.timer) { > > Aren't these two conditions redundant? > Yes, the address is in the v11 patchset. > > > + return true; > > + } > > + > > + q->tx_coal.pkt_cnt++; > > + > > + if (q->tx_coal.packets > 0 && q->tx_coal.pkt_cnt >= > q->tx_coal.packets) { > > + q->tx_coal.pkt_cnt = 0; > > + timer_del(q->tx_coal.timer); > > + return true; > > + } > > + > > + if (!timer_pending(q->tx_coal.timer)) { > > + timer_mod(q->tx_coal.timer, > > + qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + > > + q->tx_coal.usecs * 1000); /* Converted to ns */ > > + } > > + > > + return false; > > +} > > + > > +static int virtio_net_handle_coal(VirtIONet *n, uint8_t cmd, > > + struct iovec *iov, unsigned int > iov_cnt) > > +{ > > + struct virtio_net_ctrl_coal coal; > > + VirtIONetQueue *q; > > + size_t s; > > + int i; > > + > > + 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) { > > + for (i = 0; i < n->max_queue_pairs; i++) { > > + q = &n->vqs[i]; > > + q->rx_coal.usecs = le32_to_cpu(coal.max_usecs); > > + q->rx_coal.packets = le32_to_cpu(coal.max_packets); > > + q->rx_coal.pkt_cnt = 0; > > + if (q->rx_coal.usecs > 0) { > > + /* Delete old timer if exists, then create new one */ > > Nit, I'd delete that comment as it is obvious from the code. > Yes, the address is in the v11 patchset. > > > + g_clear_pointer(&q->rx_coal.timer, timer_free); > > + q->rx_coal.timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, > > + virtio_net_rx_notify, > > + q); > > + } else { > > + g_clear_pointer(&q->rx_coal.timer, timer_free); > > + } > > + } > > + } else if (cmd == VIRTIO_NET_CTRL_NOTF_COAL_TX_SET) { > > + for (i = 0; i < n->max_queue_pairs; i++) { > > + q = &n->vqs[i]; > > + q->tx_coal.usecs = le32_to_cpu(coal.max_usecs); > > + q->tx_coal.packets = le32_to_cpu(coal.max_packets); > > + q->tx_coal.pkt_cnt = 0; > > + if (q->tx_coal.usecs > 0) { > > + g_clear_pointer(&q->tx_coal.timer, timer_free); > > + q->tx_coal.timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, > > + > virtio_net_tx_notification_timer, > > + q); > > + } else { > > + g_clear_pointer(&q->tx_coal.timer, timer_free); > > + } > > + } > > + } > > + return VIRTIO_NET_OK; > > +} > > + > > static int virtio_net_handle_rx_mode(VirtIONet *n, uint8_t cmd, > > struct iovec *iov, unsigned int > iov_cnt) > > { > > @@ -1581,6 +1672,8 @@ 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)); > > @@ -2040,7 +2133,23 @@ 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 */ > > + q->rx_coal.pkt_cnt += i; > > + if (q->rx_coal.usecs == 0 || q->rx_coal.pkt_cnt >= > q->rx_coal.packets) { > > + if (q->rx_coal.timer) { > > + timer_del(q->rx_coal.timer); > > Doesn't it need to reset q->rx_coal.pkt_cnt too? > Yes, resetting the packet count is required. Doing that in below line inside virtio_net_rx_notify function. > > > + } > > + virtio_net_rx_notify(q); > > + } else { > > + if (q->rx_coal.timer) { > > + if (!timer_pending(q->rx_coal.timer)) { > > + timer_mod(q->rx_coal.timer, > > + qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + > > + q->rx_coal.usecs * 1000); /* Converted to ns > */ > > + } > > + } > > + } > > > > return size; > > > > @@ -2690,7 +2799,9 @@ static void virtio_net_tx_complete(NetClientState > *nc, ssize_t len) > > int ret; > > > > virtqueue_push(q->tx_vq, q->async_tx.elem, 0); > > - virtio_notify(vdev, q->tx_vq); > > + if (virtio_net_tx_should_notify(q)) { > > + virtio_notify(vdev, q->tx_vq); > > + } > > > > g_free(q->async_tx.elem); > > q->async_tx.elem = NULL; > > @@ -2802,7 +2913,10 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue > *q) > > > > drop: > > virtqueue_push(q->tx_vq, elem, 0); > > - virtio_notify(vdev, q->tx_vq); > > + if (virtio_net_tx_should_notify(q)) { > > + virtio_notify(vdev, q->tx_vq); > > + } > > + > > g_free(elem); > > > > if (++num_packets >= n->tx_burst) { > > @@ -2899,6 +3013,7 @@ static void virtio_net_tx_timer(void *opaque) > > if (ret == -EBUSY || ret == -EINVAL) { > > return; > > } > > + > > Useless blank change, let's remove it. > Yes, the address is in the v11 patchset. > > > /* > > * If we flush a full burst of packets, assume there are > > * more coming and immediately rearm > > @@ -3005,6 +3120,12 @@ static void virtio_net_add_queue(VirtIONet *n, > int index) > > &DEVICE(vdev)->mem_reentrancy_guard); > > > > n->vqs[index].tx_waiting = 0; > > + n->vqs[index].rx_coal.pkt_cnt = 0; > > + n->vqs[index].tx_coal.pkt_cnt = 0; > > + n->vqs[index].rx_coal.usecs = 0; > > + n->vqs[index].tx_coal.usecs = 0; > > + n->vqs[index].rx_coal.packets = 0; > > + n->vqs[index].tx_coal.packets = 0; > > n->vqs[index].n = n; > > } > > > > @@ -3017,6 +3138,11 @@ static void virtio_net_del_queue(VirtIONet *n, > int index) > > qemu_purge_queued_packets(nc); > > > > virtio_del_queue(vdev, index * 2); > > + > > + /* Cleanup coalescing timers */ > > Remove this comment as it is obvious from the code. > Yes, the address is in the v11 patchset. > > > + g_clear_pointer(&q->rx_coal.timer, timer_free); > > + g_clear_pointer(&q->tx_coal.timer, timer_free); > > + > > if (q->tx_timer) { > > timer_free(q->tx_timer); > > q->tx_timer = NULL; > > @@ -3097,6 +3223,13 @@ 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); > > + if (n->tx_timer_activate) { > > + virtio_clear_feature_ex(features, VIRTIO_NET_F_NOTF_COAL); > > + } else { > > + if (!virtio_has_feature(*features, VIRTIO_NET_F_NOTF_COAL)) { > > + *features &= ~(1ULL << VIRTIO_NET_F_NOTF_COAL); > > + } > > + } > > > > if (!peer_has_vnet_hdr(n)) { > > virtio_clear_feature_ex(features, VIRTIO_NET_F_CSUM); > > @@ -3251,6 +3384,26 @@ static int virtio_net_post_load_device(void > *opaque, int version_id) > > } > > > > virtio_net_commit_rss_config(n); > > + > > + for (i = 0; i < n->max_queue_pairs; i++) { > > + VirtIONetQueue *q = &n->vqs[i]; > > + if (q->rx_coal.usecs > 0) { > > + if (!q->rx_coal.timer) { > > Let's save one indentation by using if (q->rx_coal.usecs > 0 && > !q->rx_coal.timer). Same for tx. > Yes, the address is in the v11 patchset. > > > + q->rx_coal.timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, > > + virtio_net_rx_notify, > > + q); > > + } > > + } > > + > > + if (q->tx_coal.usecs > 0) { > > + if (!q->tx_coal.timer) { > > + q->tx_coal.timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, > > + > virtio_net_tx_notification_timer, > > + q); > > + } > > + } > > + } > > + > > return 0; > > } > > > > @@ -3270,13 +3423,49 @@ static int > virtio_net_post_load_virtio(VirtIODevice *vdev) > > return 0; > > } > > > > +static const VMStateDescription vmstate_virtio_net_coal = { > > + .name = "virtio-net-coal", > > + .version_id = 1, > > + .minimum_version_id = 1, > > + .fields = (const VMStateField[]) { > > + VMSTATE_UINT32(usecs, VirtIONetCoal), > > + VMSTATE_UINT32(packets, VirtIONetCoal), > > + VMSTATE_UINT32(pkt_cnt, VirtIONetCoal), > > + VMSTATE_END_OF_LIST() > > + }, > > +}; > > + > > +static bool virtio_net_queue_notf_coal_needed(void *opaque) > > +{ > > + VirtIONetQueue *q = opaque; > > + return q->rx_coal.usecs > 0 || q->tx_coal.usecs > 0; > > +} > > + > > +static const VMStateDescription vmstate_virtio_net_queue_notf_coal = { > > + .name = "virtio-net-queue-tx_waiting/notf-coal", > > + .version_id = 1, > > + .minimum_version_id = 1, > > + .needed = virtio_net_queue_notf_coal_needed, > > + .fields = (const VMStateField[]) { > > + VMSTATE_STRUCT(rx_coal, VirtIONetQueue, 0, > > + vmstate_virtio_net_coal, VirtIONetCoal), > > + VMSTATE_STRUCT(tx_coal, VirtIONetQueue, 0, > > + vmstate_virtio_net_coal, VirtIONetCoal), > > + VMSTATE_END_OF_LIST() > > + }, > > +}; > > + > > /* tx_waiting field of a VirtIONetQueue */ > > static const VMStateDescription vmstate_virtio_net_queue_tx_waiting = { > > .name = "virtio-net-queue-tx_waiting", > > .fields = (const VMStateField[]) { > > VMSTATE_UINT32(tx_waiting, VirtIONetQueue), > > VMSTATE_END_OF_LIST() > > - }, > > + }, > > + .subsections = (const VMStateDescription * const []) { > > + &vmstate_virtio_net_queue_notf_coal, > > + NULL > > + } > > }; > > > > static bool max_queue_pairs_gt_1(void *opaque, int version_id) > > @@ -3981,6 +4170,10 @@ static void virtio_net_device_realize(DeviceState > *dev, Error **errp) > > > > if (n->net_conf.tx && strcmp(n->net_conf.tx, "timer") == 0) { > > n->tx_timer_activate = true; > > + if (n->host_features & (1ULL << VIRTIO_NET_F_NOTF_COAL)) { > > + warn_report("virtio-net: 'vq_notf_coal' is incompatible > with 'tx=timer'," > > + "disabling notification coalescing"); > > + } > > } > > > > n->net_conf.tx_queue_size = MIN(virtio_net_max_tx_queue_size(n), > > @@ -4271,6 +4464,8 @@ 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("x-vq_notf_coal", VirtIONet, host_features, > > + VIRTIO_NET_F_NOTF_COAL, false), > > DEFINE_PROP_ON_OFF_AUTO_BIT64("hash-ipv4", VirtIONet, > > rss_data.specified_hash_types, > > VIRTIO_NET_HASH_REPORT_IPv4 - 1, > > diff --git a/include/hw/virtio/virtio-net.h > b/include/hw/virtio/virtio-net.h > > index a4eb3f407e..244c8a0224 100644 > > --- a/include/hw/virtio/virtio-net.h > > +++ b/include/hw/virtio/virtio-net.h > > @@ -155,6 +155,14 @@ typedef struct VirtioNetRssData { > > uint16_t default_queue; > > } VirtioNetRssData; > > > > +/* Interrupt notification coalescing parameters */ > > +typedef struct VirtIONetCoal { > > + uint32_t usecs; /* Timeout in microseconds */ > > + uint32_t packets; /* Packet count threshold */ > > + uint32_t pkt_cnt; /* Current packet count since last notification > */ > > + QEMUTimer *timer; /* Coalescing timer */ > > +} VirtIONetCoal; > > + > > typedef struct VirtIONetQueue { > > VirtQueue *rx_vq; > > VirtQueue *tx_vq; > > @@ -165,6 +173,8 @@ typedef struct VirtIONetQueue { > > VirtQueueElement *elem; > > } async_tx; > > struct VirtIONet *n; > > + VirtIONetCoal rx_coal; > > + VirtIONetCoal tx_coal; > > } VirtIONetQueue; > > > > struct VirtIONet { > > diff --git a/net/passt.c b/net/passt.c > > index 45440c399b..43b36ed8c5 100644 > > --- a/net/passt.c > > +++ b/net/passt.c > > @@ -52,6 +52,7 @@ static const int user_feature_bits[] = { > > VIRTIO_NET_F_GUEST_USO4, > > VIRTIO_NET_F_GUEST_USO6, > > VIRTIO_NET_F_HOST_USO, > > + VIRTIO_NET_F_NOTF_COAL, > > > > /* This bit implies RARP isn't sent by QEMU out of band */ > > VIRTIO_NET_F_GUEST_ANNOUNCE, > > diff --git a/net/tap.c b/net/tap.c > > index 57ffb09885..e6ddbc1eb1 100644 > > --- a/net/tap.c > > +++ b/net/tap.c > > @@ -63,6 +63,7 @@ static const int kernel_feature_bits[] = { > > VIRTIO_F_NOTIFICATION_DATA, > > VIRTIO_NET_F_RSC_EXT, > > VIRTIO_NET_F_HASH_REPORT, > > + VIRTIO_NET_F_NOTF_COAL, > > VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO, > > VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO, > > VHOST_INVALID_FEATURE_BIT > > diff --git a/net/vhost-user.c b/net/vhost-user.c > > index 2d0fc49b4d..f1e9b7a038 100644 > > --- a/net/vhost-user.c > > +++ b/net/vhost-user.c > > @@ -54,6 +54,7 @@ static const int user_feature_bits[] = { > > VIRTIO_NET_F_GUEST_USO4, > > VIRTIO_NET_F_GUEST_USO6, > > VIRTIO_NET_F_HOST_USO, > > + VIRTIO_NET_F_NOTF_COAL, > > > > /* This bit implies RARP isn't sent by QEMU out of band */ > > VIRTIO_NET_F_GUEST_ANNOUNCE, > > diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c > > index f1523697e2..0dcd6fb9f1 100644 > > --- a/net/vhost-vdpa.c > > +++ b/net/vhost-vdpa.c > > @@ -70,6 +70,7 @@ 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_NOTF_COAL, > > VIRTIO_NET_F_CTRL_VQ, > > VIRTIO_NET_F_GSO, > > VIRTIO_NET_F_GUEST_CSUM, > > -- > > 2.53.0 > > > >
