Re: virtio: why no full reset on virtio_set_status 0 ?

[Claudio Fontana]

On 7/29/22 16:00, Claudio Fontana wrote:
> On 7/29/22 15:21, Alex Bennée wrote:
>>
>> Claudio Fontana <cfontana@suse.de> writes:
>>
>>> On 7/29/22 12:13, Michael S. Tsirkin wrote:
>>>> On Fri, Jul 29, 2022 at 11:46:05AM +0200, Claudio Fontana wrote:
>>>>>>> @@ -2025,7 +2031,6 @@ void virtio_reset(void *opaque)
>>>>>>>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>>>>>>>      int i;
>>>>>>>  
>>>>>>> -    virtio_set_status(vdev, 0);
>>>>>>>      if (current_cpu) {
>>>>>>>          /* Guest initiated reset */
>>>>>>>          vdev->device_endian = virtio_current_cpu_endian();
>>>>>>> -- 
>>>>>>> 2.26.2
>>>>>>
>>>>>> As you say this is incomplete ... bout could you share a bit more
>>>>>> of what issue does this address?
>>>>>>
>>>>>
>>>>> Hi, the problem I am trying to address is a segfault in OVS/dpdk that 
>>>>> looks like this:
>>>>
>>>> Sorry I was not clear. What I mean is, you don't yet know why does removing
>>>> virtio_set_status call here prevent the crash in ovs, do you?
>>>>
>>>
>>> I have no idea. Trying to collect logs to figure things out, but as
>>> mentioned the logs easily hide the issue.
>>> Likely there is just more to study here.
>>
>> Given the OVS is going off on a NULL ptr deref could it just be it's not
>> handling the disabling/reenabling of the virtqueues as you pause and
>> restart properly? I could certainly imagine a backend jumping the gun to
>> read a queue going very wrong if the current queue state is disabled.
>>
> 
> In this case both the ovs buf_addr and buf_iova are NULL, which is a nice 
> case as they are more detectable,
> however I also have segfaults where the addresses are just garbage.
> 
> I wonder whether it's possible that given the fact that the guest is going 
> away without notification (SIGKILL),
> as the guest driver resets the device and communicates with QEMU, QEMU adapts 
> the state without notifying ovs,
> so ovs happily tries to dequeue data from memory that isn't there. But I am 
> just guessing.
> 
> I am still studying the qemu vhost user side and ovs/dpdk side to try to 
> understand how this whole thing works.
> 
> Thanks,
> 
> CLaudio
> 

I am pursuing this as a DPDK library issue.

It would be cool to have ovs, dpdk and vhost-user with the default test-pmd 
application somehow hooked up in a basic test
in one of these projects..

Thanks,

Claudio