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.
Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
---
hw/virtio-net.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++---------
1 files changed, 68 insertions(+), 13 deletions(-)
diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 8b652f2..3288c77 100644
--- 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;
@@ -704,16 +705,25 @@ static void virtio_net_handle_tx(VirtIODevice *vdev, VirtQueue *vq)
{
VirtIONet *n = to_virtio_net(vdev);
- if (n->tx_waiting) {
- virtio_queue_set_notification(vq, 1);
- qemu_del_timer(n->tx_timer);
- n->tx_waiting = 0;
- virtio_net_flush_tx(n, vq);
+ if (n->tx_timer) {
+ if (n->tx_waiting) {
+ virtio_queue_set_notification(vq, 1);
+ qemu_del_timer(n->tx_timer);
+ n->tx_waiting = 0;
+ virtio_net_flush_tx(n, vq);
+ } else {
+ qemu_mod_timer(n->tx_timer,
+ qemu_get_clock(vm_clock) + n->tx_timeout);
+ n->tx_waiting = 1;
+ virtio_queue_set_notification(vq, 0);
+ }
} else {
- qemu_mod_timer(n->tx_timer,
- qemu_get_clock(vm_clock) + n->tx_timeout);
+ if (unlikely(n->tx_waiting)) {
+ return;
+ }
+ virtio_queue_set_notification(n->tx_vq, 0);
+ qemu_bh_schedule(n->tx_bh);
n->tx_waiting = 1;
- virtio_queue_set_notification(vq, 0);
}
}
@@ -731,6 +741,41 @@ 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 +895,12 @@ 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;
}
@@ -939,9 +988,9 @@ 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;
if (txtimer) {
+ n->tx_timer = qemu_new_timer(vm_clock, virtio_net_tx_timer, n);
if (txtimer == 1) {
/* For convenience, 1 = "on" = predefined default, anything else
* specifies and actual timeout value */
@@ -949,6 +998,8 @@ VirtIODevice *virtio_net_init(DeviceState *dev, NICConf *conf,
} else {
n->tx_timeout = txtimer;
}
+ } else {
+ n->tx_bh = qemu_bh_new(virtio_net_tx_bh, n);
}
n->tx_burst = txburst;
n->mergeable_rx_bufs = 0;
@@ -982,8 +1033,12 @@ 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);
--
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