Re: [Qemu-devel] [PATCH] net: Forbid dealing with packets when VM is not running

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* zhanghailiang (zhang.zhanghailiang@huawei.com) wrote:
> For all NICs(except virtio-net) emulated by qemu,
> Such as e1000, rtl8139, pcnet and ne2k_pci,
> Qemu can still receive packets when VM is not running.
> If this happened in *migration's* last PAUSE VM stage,
> The new dirty RAM related to the packets will be missed,
> And this will lead serious network fault in VM.

I'd like to understand more about when exactly this happens.
migration.c:migration_thread  in the last step, takes the iothread, puts
the runstate into RUN_STATE_FINISH_MIGRATE and only then calls qemu_savevm_state_complete
before unlocking the iothread.

If that's true, how does this problem happen - can the net code
still receive packets with the iothread lock taken?
qemu_savevm_state_complete does a call to the RAM code, so I think this should get
any RAM changes that happened before the lock was taken.

I'm gently worried about threading stuff as well - is all this net code
running off fd handlers on the main thread or are there separate threads?

Dave

> To avoid this, we forbid receiving packets in generic net code when
> VM is not running. Also, when the runstate changes back to running,
> we definitely need to flush queues to get packets flowing again.
> 
> Here we implement this in the net layer:
> (1) Judge the vm runstate in qemu_can_send_packet
> (2) Add a member 'VMChangeStateEntry *vmstate' to struct NICState,
> Which will listen for VM runstate changes.
> (3) Register a handler function for VMstate change.
> When vm changes back to running, we flush all queues in the callback function.
> (4) Remove checking vm state in virtio_net_can_receive
> 
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
> ---
>  hw/net/virtio-net.c |  4 ----
>  include/net/net.h   |  2 ++
>  net/net.c           | 32 ++++++++++++++++++++++++++++++++
>  3 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 268eff9..287d762 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -839,10 +839,6 @@ static int virtio_net_can_receive(NetClientState *nc)
>      VirtIODevice *vdev = VIRTIO_DEVICE(n);
>      VirtIONetQueue *q = virtio_net_get_subqueue(nc);
>  
> -    if (!vdev->vm_running) {
> -        return 0;
> -    }
> -
>      if (nc->queue_index >= n->curr_queues) {
>          return 0;
>      }
> diff --git a/include/net/net.h b/include/net/net.h
> index ed594f9..a294277 100644
> --- a/include/net/net.h
> +++ b/include/net/net.h
> @@ -8,6 +8,7 @@
>  #include "net/queue.h"
>  #include "migration/vmstate.h"
>  #include "qapi-types.h"
> +#include "sysemu/sysemu.h"
>  
>  #define MAX_QUEUE_NUM 1024
>  
> @@ -96,6 +97,7 @@ typedef struct NICState {
>      NICConf *conf;
>      void *opaque;
>      bool peer_deleted;
> +    VMChangeStateEntry *vmstate;
>  } NICState;
>  
>  NetClientState *qemu_find_netdev(const char *id);
> diff --git a/net/net.c b/net/net.c
> index 6d930ea..21f0d48 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -242,6 +242,29 @@ NetClientState *qemu_new_net_client(NetClientInfo *info,
>      return nc;
>  }
>  
> +static void nic_vmstate_change_handler(void *opaque,
> +                                       int running,
> +                                       RunState state)
> +{
> +    NICState *nic = opaque;
> +    NetClientState *nc;
> +    int i, queues;
> +
> +    if (!running) {
> +        return;
> +    }
> +
> +    queues = MAX(1, nic->conf->peers.queues);
> +    for (i = 0; i < queues; i++) {
> +        nc = &nic->ncs[i];
> +        if (nc->receive_disabled
> +            || (nc->info->can_receive && !nc->info->can_receive(nc))) {
> +            continue;
> +        }
> +        qemu_flush_queued_packets(nc);
> +    }
> +}
> +
>  NICState *qemu_new_nic(NetClientInfo *info,
>                         NICConf *conf,
>                         const char *model,
> @@ -259,6 +282,8 @@ NICState *qemu_new_nic(NetClientInfo *info,
>      nic->ncs = (void *)nic + info->size;
>      nic->conf = conf;
>      nic->opaque = opaque;
> +    nic->vmstate = qemu_add_vm_change_state_handler(nic_vmstate_change_handler,
> +                                                    nic);
>  
>      for (i = 0; i < queues; i++) {
>          qemu_net_client_setup(&nic->ncs[i], info, peers[i], model, name,
> @@ -379,6 +404,7 @@ void qemu_del_nic(NICState *nic)
>          qemu_free_net_client(nc);
>      }
>  
> +    qemu_del_vm_change_state_handler(nic->vmstate);
>      g_free(nic);
>  }
>  
> @@ -452,6 +478,12 @@ void qemu_set_vnet_hdr_len(NetClientState *nc, int len)
>  
>  int qemu_can_send_packet(NetClientState *sender)
>  {
> +    int vmstat = runstate_is_running();
> +
> +    if (!vmstat) {
> +        return 0;
> +    }
> +
>      if (!sender->peer) {
>          return 1;
>      }
> -- 
> 1.7.12.4
> 
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK