virtio-net: Introduce a new bottom half packet TX

Based on a patch from Mark McLoughlin, this patch introduces a new
bottom half packet transmitter that avoids the latency imposed by
the tx_timer approach.  Rather than scheduling a timer when a TX
packet comes in, schedule a bottom half to be run from the iothread.
The bottom half handler first attempts to flush the queue with
notification disabled (this is where we could race with a guest
without txburst).  If we flush a full burst, reschedule immediately.
If we send short of a full burst, try to re-enable notification.
To avoid a race with TXs that may have occurred, we must then
flush again.  If we find some packets to send, the guest it probably
active, so we can reschedule again.

tx_timer and tx_bh are mutually exclusive, so we can re-use the
tx_waiting flag to indicate one or the other needs to be setup.
This allows us to seamlessly migrate between timer and bh TX
handling.

The bottom half handler becomes the new default and we add a new
tx= option to virtio-net-pci.  Usage:

-device virtio-net-pci,tx=timer # select timer mitigation vs "bh"

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/hw/s390-virtio-bus.c b/hw/s390-virtio-bus.c
index 092e65f0ca528ba911eddb3cabb4a273c2c4c8ae..784dc01b97d2e5a7f7fc48e8b4967cbc81e94b3c 100644 (file)
--- a/hw/s390-virtio-bus.c
+++ b/hw/s390-virtio-bus.c
@@ -332,6 +332,7 @@ static VirtIOS390DeviceInfo s390_virtio_net = {
                            net.txtimer, TX_TIMER_INTERVAL),
         DEFINE_PROP_INT32("x-txburst", VirtIOS390Device,
                           net.txburst, TX_BURST),
+        DEFINE_PROP_STRING("tx", VirtIOS390Device, net.tx),
         DEFINE_PROP_END_OF_LIST(),
     },
 };

diff --git a/hw/syborg_virtio.c b/hw/syborg_virtio.c
index 3c3f3b025baa2914f968fab97af57e6bd60aca48..4dfd1a87b94586a56b6e0b23f7fee06a91e71a7e 100644 (file)
--- a/hw/syborg_virtio.c
+++ b/hw/syborg_virtio.c
@@ -300,6 +300,7 @@ static SysBusDeviceInfo syborg_virtio_net_info = {
                            net.txtimer, TX_TIMER_INTERVAL),
         DEFINE_PROP_INT32("x-txburst", SyborgVirtIOProxy,
                           net.txburst, TX_BURST),
+        DEFINE_PROP_STRING("tx", SyborgVirtIOProxy, net.tx),
         DEFINE_PROP_END_OF_LIST(),
     }
 };

diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 1a4ba217877afafd3924ec182cb39d9cc4f1c678..0a9cae22c45ead87c34d5383868c379be1ace5ba 100644 (file)
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -36,6 +36,7 @@ typedef struct VirtIONet
     VirtQueue *ctrl_vq;
     NICState *nic;
     QEMUTimer *tx_timer;
+    QEMUBH *tx_bh;
     uint32_t tx_timeout;
     int32_t tx_burst;
     int tx_waiting;
@@ -700,7 +701,7 @@ static int32_t virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
     return num_packets;
 }
 
-static void virtio_net_handle_tx(VirtIODevice *vdev, VirtQueue *vq)
+static void virtio_net_handle_tx_timer(VirtIODevice *vdev, VirtQueue *vq)
 {
     VirtIONet *n = to_virtio_net(vdev);
 
@@ -717,6 +718,18 @@ static void virtio_net_handle_tx(VirtIODevice *vdev, VirtQueue *vq)
     }
 }
 
+static void virtio_net_handle_tx_bh(VirtIODevice *vdev, VirtQueue *vq)
+{
+    VirtIONet *n = to_virtio_net(vdev);
+
+    if (unlikely(n->tx_waiting)) {
+        return;
+    }
+    virtio_queue_set_notification(vq, 0);
+    qemu_bh_schedule(n->tx_bh);
+    n->tx_waiting = 1;
+}
+
 static void virtio_net_tx_timer(void *opaque)
 {
     VirtIONet *n = opaque;
@@ -731,6 +744,41 @@ static void virtio_net_tx_timer(void *opaque)
     virtio_net_flush_tx(n, n->tx_vq);
 }
 
+static void virtio_net_tx_bh(void *opaque)
+{
+    VirtIONet *n = opaque;
+    int32_t ret;
+
+    n->tx_waiting = 0;
+
+    /* Just in case the driver is not ready on more */
+    if (unlikely(!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK)))
+        return;
+
+    ret = virtio_net_flush_tx(n, n->tx_vq);
+    if (ret == -EBUSY) {
+        return; /* Notification re-enable handled by tx_complete */
+    }
+
+    /* If we flush a full burst of packets, assume there are
+     * more coming and immediately reschedule */
+    if (ret >= n->tx_burst) {
+        qemu_bh_schedule(n->tx_bh);
+        n->tx_waiting = 1;
+        return;
+    }
+
+    /* If less than a full burst, re-enable notification and flush
+     * anything that may have come in while we weren't looking.  If
+     * we find something, assume the guest is still active and reschedule */
+    virtio_queue_set_notification(n->tx_vq, 1);
+    if (virtio_net_flush_tx(n, n->tx_vq) > 0) {
+        virtio_queue_set_notification(n->tx_vq, 0);
+        qemu_bh_schedule(n->tx_bh);
+        n->tx_waiting = 1;
+    }
+}
+
 static void virtio_net_save(QEMUFile *f, void *opaque)
 {
     VirtIONet *n = opaque;
@@ -850,8 +898,12 @@ static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
     n->mac_table.first_multi = i;
 
     if (n->tx_waiting) {
-        qemu_mod_timer(n->tx_timer,
-                       qemu_get_clock(vm_clock) + n->tx_timeout);
+        if (n->tx_timer) {
+            qemu_mod_timer(n->tx_timer,
+                           qemu_get_clock(vm_clock) + n->tx_timeout);
+        } else {
+            qemu_bh_schedule(n->tx_bh);
+        }
     }
     return 0;
 }
@@ -929,7 +981,22 @@ VirtIODevice *virtio_net_init(DeviceState *dev, NICConf *conf,
     n->vdev.reset = virtio_net_reset;
     n->vdev.set_status = virtio_net_set_status;
     n->rx_vq = virtio_add_queue(&n->vdev, 256, virtio_net_handle_rx);
-    n->tx_vq = virtio_add_queue(&n->vdev, 256, virtio_net_handle_tx);
+
+    if (net->tx && strcmp(net->tx, "timer") && strcmp(net->tx, "bh")) {
+        fprintf(stderr, "virtio-net: "
+                "Unknown option tx=%s, valid options: \"timer\" \"bh\"\n",
+                net->tx);
+        fprintf(stderr, "Defaulting to \"bh\"\n");
+    }
+
+    if (net->tx && !strcmp(net->tx, "timer")) {
+        n->tx_vq = virtio_add_queue(&n->vdev, 256, virtio_net_handle_tx_timer);
+        n->tx_timer = qemu_new_timer(vm_clock, virtio_net_tx_timer, n);
+        n->tx_timeout = net->txtimer;
+    } else {
+        n->tx_vq = virtio_add_queue(&n->vdev, 256, virtio_net_handle_tx_bh);
+        n->tx_bh = qemu_bh_new(virtio_net_tx_bh, n);
+    }
     n->ctrl_vq = virtio_add_queue(&n->vdev, 64, virtio_net_handle_ctrl);
     qemu_macaddr_default_if_unset(&conf->macaddr);
     memcpy(&n->mac[0], &conf->macaddr, sizeof(n->mac));
@@ -939,9 +1006,7 @@ VirtIODevice *virtio_net_init(DeviceState *dev, NICConf *conf,
 
     qemu_format_nic_info_str(&n->nic->nc, conf->macaddr.a);
 
-    n->tx_timer = qemu_new_timer(vm_clock, virtio_net_tx_timer, n);
     n->tx_waiting = 0;
-    n->tx_timeout = net->txtimer;
     n->tx_burst = net->txburst;
     n->mergeable_rx_bufs = 0;
     n->promisc = 1; /* for compatibility */
@@ -974,8 +1039,12 @@ void virtio_net_exit(VirtIODevice *vdev)
     qemu_free(n->mac_table.macs);
     qemu_free(n->vlans);
 
-    qemu_del_timer(n->tx_timer);
-    qemu_free_timer(n->tx_timer);
+    if (n->tx_timer) {
+        qemu_del_timer(n->tx_timer);
+        qemu_free_timer(n->tx_timer);
+    } else {
+        qemu_bh_delete(n->tx_bh);
+    }
 
     virtio_cleanup(&n->vdev);
     qemu_del_vlan_client(&n->nic->nc);

diff --git a/hw/virtio-net.h b/hw/virtio-net.h
index a2d1545e4cc5f7c409bbde9aba641a63e6a78b3c..8af9a1ce55fd8d8814d02cd8906e7c7c6d76c194 100644 (file)
--- a/hw/virtio-net.h
+++ b/hw/virtio-net.h
@@ -60,6 +60,7 @@ typedef struct virtio_net_conf
 {
     uint32_t txtimer;
     int32_t txburst;
+    char *tx;
 } virtio_net_conf;
 
 /* Maximum packet size we can receive from tap device: header + 64k */

diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index 3a5b3e6b237b3ba4e742abf9a60491a8e3c2d2f5..86e6b0a5617c2bc84c18b5cc9eaf6773e69c6beb 100644 (file)
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -695,6 +695,7 @@ static PCIDeviceInfo virtio_info[] = {
                                net.txtimer, TX_TIMER_INTERVAL),
             DEFINE_PROP_INT32("x-txburst", VirtIOPCIProxy,
                               net.txburst, TX_BURST),
+            DEFINE_PROP_STRING("tx", VirtIOPCIProxy, net.tx),
             DEFINE_PROP_END_OF_LIST(),
         },
         .qdev.reset = virtio_pci_reset,