Re: [BUG] vhost-vdpa: qemu-system-s390x crashes with second virtio-net-ccw device

[Jason Wang <jasowang@redhat.com>]

On 2020/7/27 下午2:43, Cornelia Huck wrote:
> On Sat, 25 Jul 2020 08:40:07 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> On 2020/7/24 下午11:34, Cornelia Huck wrote:
>>> On Fri, 24 Jul 2020 11:17:57 -0400
>>> "Michael S. Tsirkin"<mst@redhat.com>  wrote:
>>>   
>>>> On Fri, Jul 24, 2020 at 04:56:27PM +0200, Cornelia Huck wrote:
>>>>> On Fri, 24 Jul 2020 09:30:58 -0400
>>>>> "Michael S. Tsirkin"<mst@redhat.com>  wrote:
>>>>>       
>>>>>> On Fri, Jul 24, 2020 at 03:27:18PM +0200, Cornelia Huck wrote:
>>>>>>> When I start qemu with a second virtio-net-ccw device (i.e. adding
>>>>>>> -device virtio-net-ccw in addition to the autogenerated device), I get
>>>>>>> a segfault. gdb points to
>>>>>>>
>>>>>>> #0  0x000055d6ab52681d in virtio_net_get_config (vdev=<optimized out>,
>>>>>>>       config=0x55d6ad9e3f80 "RT") at /home/cohuck/git/qemu/hw/net/virtio-net.c:146
>>>>>>> 146	    if (nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
>>>>>>>
>>>>>>> (backtrace doesn't go further)
>>>>> The core was incomplete, but running under gdb directly shows that it
>>>>> is just a bog-standard config space access (first for that device).
>>>>>
>>>>> The cause of the crash is that nc->peer is not set... no idea how that
>>>>> can happen, not that familiar with that part of QEMU. (Should the code
>>>>> check, or is that really something that should not happen?)
>>>>>
>>>>> What I don't understand is why it is set correctly for the first,
>>>>> autogenerated virtio-net-ccw device, but not for the second one, and
>>>>> why virtio-net-pci doesn't show these problems. The only difference
>>>>> between -ccw and -pci that comes to my mind here is that config space
>>>>> accesses for ccw are done via an asynchronous operation, so timing
>>>>> might be different.
>>>> Hopefully Jason has an idea. Could you post a full command line
>>>> please? Do you need a working guest to trigger this? Does this trigger
>>>> on an x86 host?
>>> Yes, it does trigger with tcg-on-x86 as well. I've been using
>>>
>>> s390x-softmmu/qemu-system-s390x -M s390-ccw-virtio,accel=tcg -cpu qemu,zpci=on
>>> -m 1024 -nographic -device virtio-scsi-ccw,id=scsi0,devno=fe.0.0001
>>> -drive file=/path/to/image,format=qcow2,if=none,id=drive-scsi0-0-0-0
>>> -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,bootindex=1
>>> -device virtio-net-ccw
>>>
>>> It seems it needs the guest actually doing something with the nics; I
>>> cannot reproduce the crash if I use the old advent calendar moon buggy
>>> image and just add a virtio-net-ccw device.
>>>
>>> (I don't think it's a problem with my local build, as I see the problem
>>> both on my laptop and on an LPAR.)
>>
>> It looks to me we forget the check the existence of peer.
>>
>> Please try the attached patch to see if it works.
> Thanks, that patch gets my guest up and running again. So, FWIW,
>
> Tested-by: Cornelia Huck <cohuck@redhat.com>
>
> Any idea why this did not hit with virtio-net-pci (or the autogenerated
> virtio-net-ccw device)?


It can be hit with virtio-net-pci as well (just start without peer).

For autogenerated virtio-net-cww, I think the reason is that it has 
already had a peer set.

Thanks


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