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[80.230.79.236]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6a9882a8510sm2255625a12.21.2026.09.10.00.52.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Sep 2026 00:52:13 -0700 (PDT) Date: Thu, 10 Sep 2026 03:52:09 -0400 From: "Michael S. Tsirkin" To: Koushik Dutta Cc: qemu-devel@nongnu.org, Eugenio =?iso-8859-1?Q?P=E9rez?= , Stefano Garzarella , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Zhao Liu , Jason Wang Subject: Re: [PATCH v10 2/2] [PATCH 2/2] Implement VirtIO Network Notification Coalescing (VIRTIO_NET_F_NOTF_COAL). This allows guests to reduce interrupt overhead by configuring coalescing parameters via ethtool -C for both RX and TX paths. Message-ID: <20260910034751-mutt-send-email-mst@kernel.org> References: <20260716134421.3765533-1-kdutta@redhat.com> <20260716134421.3765533-3-kdutta@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260716134421.3765533-3-kdutta@redhat.com> Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org subject is unreasonably long here On Thu, Jul 16, 2026 at 07:14:21PM +0530, Koushik Dutta 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. > > Adds the `x-vq_notf_coal=on` device parameter to dynamically enable > virtqueue notification coalescing at VM startup. we really do not want to train users to use x- properties. > 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 > --- > 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) { > + 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 */ > + 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); > + } > + 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; > } > + > /* > * 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 */ > + 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) { > + 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); > + } > + } > + } > + Not QEMU_CLOCK_VIRTUAL_RT? > 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 please do not add these bits until someone actually implements this. -- MST