Re: [Qemu-devel] How to lock-up your tap-based VM network

[Jan Kiszka <jan.kiszka@siemens.com>]

Paul Brook wrote:
>> Paul Brook wrote:
>>>> A major reason for this deadlock could likely be removed by shutting
>>>> down the tap (if peered) or dropping packets in user space (in case of
>>>> vlan) when a NIC is stopped or otherwise shut down. Currently most (if
>>>> not all) NIC models seem to signal both "queue full" and "RX disabled"
>>>> via !can_receive().
>>> No. A disabled device should return true from can_recieve, then discard
>>> the packets in its receive callback. Failure to do so is a bug in the
>>> device. It looks like the virtio-net device may be buggy.
>> That's not a virtio-only issue. In fact, we ran into this over pcnet,
>> and a quick check of other popular PCI NIC models (except for rtl8139)
>> revealed the same picture: They only report can_receive if their
>> receiver unit is up and ready (some also include the queue state, but
>> that's an "add-on").
> 
> If so these are also buggy.
> 
>> I think it's clear why: "can_receive" strongly suggests that a suspended
>> receiver should make the model return false. If we want to keep this
>> handler, it should be refactored to something like "queue_full".
> 
> I don't see a need to refactor anything.  You just need to fix the devices 
> that incorrectly return false when their RX engine is disabled.

"can_receive" is a misleading name. NIC model developers will continue
to make mistakes when implementing this function. And I don't think we
want such implementations all around:

static int rtl8139_can_receive(VLANClientState *nc)
{
    RTL8139State *s = DO_UPCAST(NICState, nc, nc)->opaque;
    int avail;

    /* Receive (drop) packets if card is disabled.  */
    if (!s->clock_enabled)
      return 1;
    if (!rtl8139_receiver_enabled(s))
      return 1;

    if (rtl8139_cp_receiver_enabled(s)) {
        /* ??? Flow control not implemented in c+ mode.
           This is a hack to work around slirp deficiencies anyway.  */
        return 1;
    } else {
        avail = MOD2(s->RxBufferSize + s->RxBufPtr - s->RxBufAddr,
                     s->RxBufferSize);
        return (avail == 0 || avail >= 1514);
    }
}

Jan

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