Re: [Qemu-devel] [PATCH] virtio_net: flush uncompleted TX on reset

[Nageswara R Sastry]

Greg Kurz <address@hidden> wrote on 08/03/2018 12:27:37 AM:

> From: Greg Kurz <address@hidden>
> To: address@hidden
> Cc: "Michael S. Tsirkin" <address@hidden>, Jason Wang 
> <address@hidden>, R Nageswara Sastry <address@hidden>
> Date: 08/03/2018 12:27 AM
> Subject: [PATCH] virtio_net: flush uncompleted TX on reset
> 
> If the backend could not transmit a packet right away for some reason,
> the packet is queued for asynchronous sending. The corresponding vq
> element is tracked in the async_tx.elem field of the VirtIONetQueue,
> for later freeing when the transmission is complete.
> 
> If a reset happens before completion, virtio_net_tx_complete() will push
> async_tx.elem back to the guest anyway, and we end up with the inuse 
flag
> of the vq being equal to -1. The next call to virtqueue_pop() is then
> likely to fail with "Virtqueue size exceeded".
> 
> This can be reproduced easily by starting a guest without a net backend,
> doing a system reset when it is booted, and finally snapshotting it.
> 
> The appropriate fix is to ensure that such an asynchronous transmission
> cannot survive a device reset. So for all queues, we first try to send
> the packet again, and eventually we purge it if the backend still could
> not deliver it.
> 
> Reported-by: R. Nageswara Sastry <address@hidden>

Tested-by: R. Nageswara Sastry <address@hidden>

> Buglink: https://urldefense.proofpoint.com/v2/url?
> 
u=https-3A__github.com_open-2Dpower-2Dhost-2Dos_qemu_issues_37&d=DwICaQ&c=jf_iaSHvJObTbx-
> siA1ZOg&r=mxAxqGE8eb0FlPPFjDZkTNaoci-GdQbkJayE4r-
> 
wzYY&m=hPoY6b601IXJbUV2uh22jBrnYuByNQpwi1d7gvN4yZs&s=Ic3NN3mM_Nv1gAJ7dY22-
> ebnJsG7c0yNkbThX8Tu6xg&e=
> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  hw/net/virtio-net.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 188744e17d57..eea3cdb2c700 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -422,6 +422,7 @@ static RxFilterInfo 
> *virtio_net_query_rxfilter(NetClientState *nc)
>  static void virtio_net_reset(VirtIODevice *vdev)
>  {
>      VirtIONet *n = VIRTIO_NET(vdev);
> +    int i;
> 
>      /* Reset back to compatibility mode */
>      n->promisc = 1;
> @@ -445,6 +446,16 @@ static void virtio_net_reset(VirtIODevice *vdev)
>      memcpy(&n->mac[0], &n->nic->conf->macaddr, sizeof(n->mac));
>      qemu_format_nic_info_str(qemu_get_queue(n->nic), n->mac);
>      memset(n->vlans, 0, MAX_VLAN >> 3);
> +
> +    /* Flush any async TX */
> +    for (i = 0;  i < n->max_queues; i++) {
> +        NetClientState *nc = qemu_get_subqueue(n->nic, i);
> +
> +        if (!qemu_net_queue_flush(nc->peer->incoming_queue)) {
> +            qemu_net_queue_purge(nc->peer->incoming_queue, nc);
> +        }
> +        assert(!virtio_net_get_subqueue(nc)->async_tx.elem);
> +    }
>  }
> 
>  static void peer_test_vnet_hdr(VirtIONet *n)
> 


With out patch:
(qemu) system_reset
(qemu) savevm 1
Virtqueue size exceeded
(qemu) loadvm 1
VQ 1 size 0x100 < last_avail_idx 0x0 - used_idx 0x1
Failed to load virtio-net:virtio
error while loading state for instance 0x0 of device
'address@hidden:00.0/virtio-net'
Error -1 while loading VM state

With patch:
(qemu) system_reset
(qemu) savevm 1
(qemu) loadvm 1