Re: [PATCH] vhost: fix segfault on bad descriptor address.

[Ilya Maximets <i.maximets@samsung.com>]

On 11.07.2016 14:05, Yuanhan Liu wrote:
> On Mon, Jul 11, 2016 at 12:50:24PM +0300, Ilya Maximets wrote:
>> 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?
> 
> Hmm, really? You are sure that data flows in your 2nd queue after those
> commands? From what I know is that your patch just avoid a crash, but
> does not fix it.

Oh, sorry. Yes, it doesn't work. With my patch applied I have a QEMU hang.

>>> 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").
> 
> Seems yes.
> 
> 	--yliu
> 
>