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7:43, Michael S. Tsirkin wrote:
> On Mon, Jul 27, 2020 at 04:51:23PM +0800, Jason Wang wrote:
>> On 2020/7/27 下午4:41, Cornelia Huck wrote:
>>> On Mon, 27 Jul 2020 15:38:12 +0800
>>> Jason Wang<jasowang@redhat.com>  wrote:
>>>
>>>> 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).
>>> Hm, I had not been able to reproduce the crash with a 'naked' -device
>>> virtio-net-pci. But checking seems to be the right idea anyway.
>> Sorry for being unclear, I meant for networking part, you just need start
>> without peer, and you need a real guest (any Linux) that is trying to access
>> the config space of virtio-net.
>>
>> Thanks
> A pxe guest will do it, but that doesn't support ccw, right?


Yes, it depends on the cli actually.


>
> I'm still unclear why this triggers with ccw but not pci -
> any idea?


I don't test pxe but I can reproduce this with pci (just start a linux 
guest without a peer).

Thanks