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

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

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").

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".

But before starting any refactoring endeavor: Do we have a consensus on
the direction? Refactor can_receive to queue_full? Or even drop it?

Jan

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