Re: [PATCH v8] virtio-net: Fix network stall at the host side waiting for kick

[Michael S. Tsirkin]

On Fri, Jul 05, 2024 at 06:05:02PM +0800, Wencheng Yang wrote:
> From: thomas <east.moutain.yang@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, let it return the shadow_avail_idx of the virtqueue
>    on success.
> 2. Add a new API: virtio_queue_enable_notification_and_check(),
>    it takes the return value of virtqueue_get_avail_bytes() as
>    input arg, enables notification firstly, then checks whether
>    the guest has added some buffers or not since last check of
>    available buffers, 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.yang@gmail.com>
> ---
> Changelog:
> 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        | 34 +++++++++++++++--------
>  hw/virtio/virtio.c         | 56 ++++++++++++++++++++++++++++++++++++--
>  include/hw/virtio/virtio.h | 12 ++++++--
>  3 files changed, 85 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 9c7e85caea..a652aa3a16 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1641,24 +1641,34 @@ static bool virtio_net_can_receive(NetClientState *nc)
>  
>  static int virtio_net_has_buffers(VirtIONetQueue *q, int bufsize)
>  {
> +    int shadow_idx;

> +    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) {
> +        shadow_idx = virtqueue_get_avail_bytes(q->rx_vq, &in_bytes, NULL,
> +                                               bufsize, 0);
> +        /* invalid shadow idx */

No. "failure to get avail bytes"

> +        if (shadow_idx < 0) {
> +            return 0;
> +        }
> +
> +        /* buffer is enough, disable notifiaction */
> +        if (bufsize <= in_bytes) {
> +            break;
> +        }
> +
> +        if (virtio_queue_enable_notification_and_check(q->rx_vq,
> +                                                       
> (unsigned)shadow_idx)) {
> +            /* 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..d04f4d9b2e 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -745,6 +745,56 @@ 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,
> +                                                unsigned shadow_idx)
> +{
> +    virtio_queue_set_notification(vq, 1);
> +
> +    return virtio_queue_poll(vq, shadow_idx);
> +}
> +
>  static void virtqueue_unmap_sg(VirtQueue *vq, const VirtQueueElement *elem,
>                                 unsigned int len)
>  {
> @@ -1332,7 +1382,7 @@ err:
>      goto done;
>  }
>  
> -void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> +int virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>                                 unsigned int *out_bytes,
>                                 unsigned max_in_bytes, unsigned max_out_bytes)
>  {
> @@ -1367,7 +1417,7 @@ 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 +1425,8 @@ 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..c4ce7b544e 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -271,9 +271,9 @@ 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);

Document: 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 +307,12 @@ int virtio_queue_ready(VirtQueue *vq);
>  
>  int virtio_queue_empty(VirtQueue *vq);
>  
> +/**
> + * enable notification and check whether guest has added some
> + * buffers since last sync of shadow_avail_idx from the queue

No.  Since last call to virtqueue_get_avail_bytes.


> + */
> +bool virtio_queue_enable_notification_and_check(VirtQueue *vq,
> +                                                unsigned shadow_idx);

So just pass int here. you can just assert( >= 0) insternally.

And, it does not matter that it is a shadow.
Call it "opaque" please.
And document: value returned from virtqueue_get_avail_bytes.


>  /* Host binding interface.  */
>  
>  uint32_t virtio_config_readb(VirtIODevice *vdev, uint32_t addr);
> -- 
> 2.39.0