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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 27, 2020 at 08:44:09PM +0800, Jason Wang wrote:
> 
> 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
> 

Might be a good addition to a unit test. Not sure what would the
test do exactly: just make sure guest runs? Looks like a lot of work
for an empty test ... maybe we can poke at the guest config with
qtest commands at least.

-- 
MST