From: thomas <east.moutain.y...@gmail.com>
Patch 06b12970174 ("virtio-net: fix network stall under load")
added double-check to test whether the available buffer size
can satisfy the request or not, in case the guest has added
some buffers to the avail ring simultaneously after the first
check. It will be lucky if the available buffer size becomes
okay after the double-check, then the host can send the packet
to the guest. If the buffer size still can't satisfy the request,
even if the guest has added some buffers, viritio-net would
stall at the host side forever.
The patch enables notification and checks whether the guest has
added some buffers since last check of available buffers when
the available buffers are insufficient. If no buffer is added,
return false, else recheck the available buffers in the loop.
If the available buffers are sufficient, disable notification
and return true.
Changes:
1. Change the return type of virtqueue_get_avail_bytes() from void
to int, it returns an opaque that represents the shadow_avail_idx
of the virtqueue on success, else -1 on error.
2. Add a new API: virtio_queue_enable_notification_and_check(),
it takes an opaque as input arg which is returned from
virtqueue_get_avail_bytes(). It enables notification firstly,
then checks whether the guest has added some buffers since
last check of available buffers or not by virtio_queue_poll(),
return ture if yes.
The patch also reverts patch "06b12970174".
The case below can reproduce the stall.
Guest 0
+--------+
| iperf |
---------------> | server |
Host | +--------+
+--------+ | ...
| iperf |----
| client |---- Guest n
+--------+ | +--------+
| | iperf |
---------------> | server |
+--------+
Boot many guests from qemu with virtio network:
qemu ... -netdev tap,id=net_x \
-device virtio-net-pci-non-transitional,\
iommu_platform=on,mac=xx:xx:xx:xx:xx:xx,netdev=net_x
Each guest acts as iperf server with commands below:
iperf3 -s -D -i 10 -p 8001
iperf3 -s -D -i 10 -p 8002
The host as iperf client:
iperf3 -c guest_IP -p 8001 -i 30 -w 256k -P 20 -t 40000
iperf3 -c guest_IP -p 8002 -i 30 -w 256k -P 20 -t 40000
After some time, the host loses connection to the guest,
the guest can send packet to the host, but can't receive
packet from the host.
It's more likely to happen if SWIOTLB is enabled in the guest,
allocating and freeing bounce buffer takes some CPU ticks,
copying from/to bounce buffer takes more CPU ticks, compared
with that there is no bounce buffer in the guest.
Once the rate of producing packets from the host approximates
the rate of receiveing packets in the guest, the guest would
loop in NAPI.
receive packets ---
| |
v |
free buf virtnet_poll
| |
v |
add buf to avail ring ---
|
| need kick the host?
| NAPI continues
v
receive packets ---
| |
v |
free buf virtnet_poll
| |
v |
add buf to avail ring ---
|
v
... ...
On the other hand, the host fetches free buf from avail
ring, if the buf in the avail ring is not enough, the
host notifies the guest the event by writing the avail
idx read from avail ring to the event idx of used ring,
then the host goes to sleep, waiting for the kick signal
from the guest.
Once the guest finds the host is waiting for kick singal
(in virtqueue_kick_prepare_split()), it kicks the host.
The host may stall forever at the sequences below:
Host Guest
------------ -----------
fetch buf, send packet receive packet ---
... ... |
fetch buf, send packet add buf |
... add buf virtnet_poll
buf not enough avail idx-> add buf |
read avail idx add buf |
add buf ---
receive packet ---
write event idx ... |
wait for kick add buf virtnet_poll
... |
---
no more packet, exit NAPI
In the first loop of NAPI above, indicated in the range of
virtnet_poll above, the host is sending packets while the
guest is receiving packets and adding buffers.
step 1: The buf is not enough, for example, a big packet
needs 5 buf, but the available buf count is 3.
The host read current avail idx.
step 2: The guest adds some buf, then checks whether the
host is waiting for kick signal, not at this time.
The used ring is not empty, the guest continues
the second loop of NAPI.
step 3: The host writes the avail idx read from avail
ring to used ring as event idx via
virtio_queue_set_notification(q->rx_vq, 1).
step 4: At the end of the second loop of NAPI, recheck
whether kick is needed, as the event idx in the
used ring written by the host is beyound the
range of kick condition, the guest will not
send kick signal to the host.
Fixes: 06b12970174 ("virtio-net: fix network stall under load")
Signed-off-by: Wencheng Yang <east.moutain.y...@gmail.com>
---
Changelog:
v9:
- Add document
- Pass opaque returned from virtqueue_get_avail_bytes() to
virtio_queue_set_notification_and_check() as arg
v8:
- Change virtqueue_get_avail_bytes() return type from void
to int, it returns shadow_avail_idx on success.
- virtio_queue_set_notification_and_check() accepts two args,
the second arg is the shadow idx retruned from
virtqueue_get_avail_bytes()
- Add function virtio_queue_poll(), it accepts shadow idx
returned from virtqueue_get_avail_bytes() as the second
arg, and tells whether guest had add some buffers since
last check of available buffers.
v7:
- Add function virtio_queue_set_notification_and_check()
- Restore the function sequence introduce in v6
v6:
- Take packed packed queue into cosideration
- Adjust function sequence to fix compilation issue
v5:
- Modify return type of virtio_queue_set_notification() to
bool to indicate whether the guest has added some buffers
after last check of avail idx
- Loop in virtio_net_has_buffers() if the available buffers
are not sufficient and the guest has added some buffers.
- Revert patch "06b12970174"
- Update the subject
v4:
- Correct spelling mistake in the subject
- Describe the issue that virtio-net is blocked at host side
v3:
- Add virtio-net tag in the subject
- Refine commit log
v2:
- Add SOB tag at the end of the commit message
- Place Fixes tag at the end of the commit message
v1:
- Initial patch
hw/net/virtio-net.c | 28 ++++++++++-------
hw/virtio/virtio.c | 64 +++++++++++++++++++++++++++++++++++---
include/hw/virtio/virtio.h | 22 +++++++++++--
3 files changed, 95 insertions(+), 19 deletions(-)
diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 9c7e85caea..b1a10e0548 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1641,24 +1641,28 @@ static bool virtio_net_can_receive(NetClientState *nc)
static int virtio_net_has_buffers(VirtIONetQueue *q, int bufsize)
{
+ int opaque;
+ unsigned int in_bytes;
VirtIONet *n = q->n;
- if (virtio_queue_empty(q->rx_vq) ||
- (n->mergeable_rx_bufs &&
- !virtqueue_avail_bytes(q->rx_vq, bufsize, 0))) {
- virtio_queue_set_notification(q->rx_vq, 1);
-
- /* To avoid a race condition where the guest has made some buffers
- * available after the above check but before notification was
- * enabled, check for available buffers again.
- */
- if (virtio_queue_empty(q->rx_vq) ||
- (n->mergeable_rx_bufs &&
- !virtqueue_avail_bytes(q->rx_vq, bufsize, 0))) {
+
+ while (virtio_queue_empty(q->rx_vq) || n->mergeable_rx_bufs) {
+ opaque = virtqueue_get_avail_bytes(q->rx_vq, &in_bytes, NULL,
+ bufsize, 0);
+ /* buffer is enough, disable notifiaction */
+ if (bufsize <= in_bytes) {
+ break;
+ }
+
+ if (virtio_queue_enable_notification_and_check(q->rx_vq, opaque)) {
+ /* guest has added some buffers, try again */
+ continue;
+ } else {
return 0;
}
}
virtio_queue_set_notification(q->rx_vq, 0);
+
return 1;
}
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 893a072c9d..8314de45dd 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -745,6 +745,60 @@ int virtio_queue_empty(VirtQueue *vq)
}
}
+static bool virtio_queue_split_poll(VirtQueue *vq, unsigned shadow_idx)
+{
+ if (unlikely(!vq->vring.avail)) {
+ return false;
+ }
+
+ return (uint16_t)shadow_idx != vring_avail_idx(vq);
+}
+
+static bool virtio_queue_packed_poll(VirtQueue *vq, unsigned shadow_idx)
+{
+ VRingPackedDesc desc;
+ VRingMemoryRegionCaches *caches;
+
+ if (unlikely(!vq->vring.desc)) {
+ return false;
+ }
+
+ caches = vring_get_region_caches(vq);
+ if (!caches) {
+ return false;
+ }
+
+ vring_packed_desc_read(vq->vdev, &desc, &caches->desc,
+ shadow_idx, true);
+
+ return is_desc_avail(desc.flags, vq->shadow_avail_wrap_counter);
+}
+
+static bool virtio_queue_poll(VirtQueue *vq, unsigned shadow_idx)
+{
+ if (virtio_device_disabled(vq->vdev)) {
+ return false;
+ }
+
+ if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
+ return virtio_queue_packed_poll(vq, shadow_idx);
+ } else {
+ return virtio_queue_split_poll(vq, shadow_idx);
+ }
+}
+
+bool virtio_queue_enable_notification_and_check(VirtQueue *vq,
+ int opaque)
+{
+ virtio_queue_set_notification(vq, 1);
+
+ if (opaque >= 0) {
+ return virtio_queue_poll(vq, (unsigned)opaque);
+ } else {
+ return false;
+ }
+}
+
static void virtqueue_unmap_sg(VirtQueue *vq, const VirtQueueElement *elem,
unsigned int len)
{
@@ -1332,9 +1386,9 @@ err:
goto done;
}
-void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
- unsigned int *out_bytes,
- unsigned max_in_bytes, unsigned max_out_bytes)
+int virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
+ unsigned int *out_bytes, unsigned max_in_bytes,
+ unsigned max_out_bytes)
{
uint16_t desc_size;
VRingMemoryRegionCaches *caches;
@@ -1367,7 +1421,7 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned
int *in_bytes,
caches);
}
- return;
+ return (int)vq->shadow_avail_idx;
err:
if (in_bytes) {
*in_bytes = 0;
@@ -1375,6 +1429,8 @@ err:
if (out_bytes) {
*out_bytes = 0;
}
+
+ return -1;
}
int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 7d5ffdc145..ecbacdc2df 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -271,9 +271,14 @@ void qemu_put_virtqueue_element(VirtIODevice *vdev,
QEMUFile *f,
VirtQueueElement *elem);
int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
unsigned int out_bytes);
-void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
- unsigned int *out_bytes,
- unsigned max_in_bytes, unsigned max_out_bytes);
+/**
+ * return <0 on error or an opaque >=0 to pass to
+ * virtio_queue_enable_notification_and_check
+ * on asuccess
+ */
+int virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
+ unsigned int *out_bytes, unsigned max_in_bytes,
+ unsigned max_out_bytes);
void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq);
void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
@@ -307,6 +312,17 @@ int virtio_queue_ready(VirtQueue *vq);
int virtio_queue_empty(VirtQueue *vq);
+/**
+ * enable notification and check whether guest has added some
+ * buffers since last call to virtqueue_get_avail_bytes.
+ *
+ * @opaque: value returned from virtqueue_get_avail_bytes
+ */
+bool virtio_queue_enable_notification_and_check(VirtQueue *vq,
+ int opaque);
+
+void virtio_queue_set_shadow_avail_idx(VirtQueue *vq, uint16_t idx);
+
/* Host binding interface. */
uint32_t virtio_config_readb(VirtIODevice *vdev, uint32_t addr);
--
2.39.0
