[dpdk-dev] [PATCH] vhost: fix segfault on bad descriptor address.

[Ilya Maximets]

On 11.07.2016 11:38, Yuanhan Liu wrote:
> On Sun, Jul 10, 2016 at 09:17:31PM +0800, Yuanhan Liu wrote:
>> On Fri, Jul 08, 2016 at 02:48:56PM +0300, Ilya Maximets wrote:
>>>
>>> Another point is that crash constantly happens on queue_id=3 (second RX queue) in
>>> my scenario. It is newly allocated virtqueue while reconfiguration from rxq=1 to
>>> rxq=2.
>>
>> That's a valuable message: what's your DPDK HEAD commit while triggering
>> this issue?

fbfd99551ca3 ("mbuf: add raw allocation function")

> 
> I guess I have understood what goes wrong in you case.
> 
> I would guess that your vhost has 2 queues (here I mean queue-pairs,
> including one Tx and Rx queue; below usage is the same) configured,
> so does to your QEMU. However, you just enabled 1 queue while starting
> testpmd inside the guest, and you want to enable 2 queues by running
> following testpmd commands:
> 
>     stop
>     port stop all
>     port config all rxq 2
>     port config all txq 2
>     port start all
> 
> Badly, that won't work for current virtio PMD implementation, and what's
> worse, it triggers a vhost crash, the one you saw.
> 
> Here is how it comes. Since you just enabled 1 queue while starting
> testpmd, it will setup 1 queue only, meaning only one queue's **valid**
> information will be sent to vhost. You might see SET_VRING_ADDR
> (and related vhost messages) for the other queue as well, but they
> are just the dummy messages: they don't include any valid/real
> information about the 2nd queue: the driver don't setup it after all.
> 
> So far, so good. It became broken when you run above commands. Those
> commands do setup for the 2nd queue, however, they failed to trigger
> the QEMU virtio device to start the vhost-user negotiation, meaning
> no SET_VRING_ADDR will be sent for the 2nd queue, leaving vhost
> untold and not updated.
> 
> What's worse, above commands trigger the QEMU to send SET_VRING_ENABLE
> messages, to enable all the vrings. And since the vrings for the 2nd
> queue are not properly configured, the crash happens.

Hmm, why 2nd queue works properly with my fix to vhost in this case?
 
> So maybe we should do virtio reset on port start?

I guess it was removed by this patch:
a85786dc816f ("virtio: fix states handling during initialization").