Re: [Qemu-devel] [PATCH 0/4] Tweaks around virtio-blk start/stop

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/03/2016 12:56, Cornelia Huck wrote:
> On Wed, 16 Mar 2016 12:48:22 +0100
> Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
>>
>>
>> On 16/03/2016 12:32, Cornelia Huck wrote:
>>> On Wed, 16 Mar 2016 12:09:02 +0100
>>> Paolo Bonzini <pbonzini@redhat.com> wrote:
>>>
>>>> On 16/03/2016 11:49, Christian Borntraeger wrote:
>>>
>>>>> #3  0x00000000800b713e in virtio_blk_data_plane_start (s=0xba232d80) at /home/cborntra/REPOS/qemu/hw/block/dataplane/virtio-blk.c:224
>>>>> #4  0x00000000800b4ea0 in virtio_blk_handle_output (vdev=0xb9eee7e8, vq=0xba305270) at /home/cborntra/REPOS/qemu/hw/block/virtio-blk.c:590
>>>>> #5  0x00000000800ef3dc in virtio_queue_notify_vq (vq=0xba305270) at /home/cborntra/REPOS/qemu/hw/virtio/virtio.c:1095
>>>>> #6  0x00000000800f1c9c in virtio_queue_host_notifier_read (n=0xba3052c8) at /home/cborntra/REPOS/qemu/hw/virtio/virtio.c:1785
>>>>> #7  0x00000000800f1e14 in virtio_queue_set_host_notifier_fd_handler (vq=0xba305270, assign=false, set_handler=false) at /home/cborntra/REPOS/qemu/hw/virtio/virtio.c:1817
>>>>> #8  0x0000000080109c50 in virtio_ccw_set_guest2host_notifier (dev=0xb9eed6a0, n=0, assign=false, set_handler=false) at /home/cborntra/REPOS/qemu/hw/s390x/virtio-ccw.c:97
>>>>> #9  0x0000000080109ef2 in virtio_ccw_stop_ioeventfd (dev=0xb9eed6a0) at /home/cborntra/REPOS/qemu/hw/s390x/virtio-ccw.c:154
>>>>
>>>> One bug is here: virtio_ccw_stop_ioeventfd, in this case, should pass
>>>> assign=true to virtio_ccw_set_guest2host_notifier.  (Assuming my
>>>> understanding of assign=true is correct; I think it means "I'm going to
>>>> set up another host notifier handler").
>>>
>>> Hm... I copied these semantics from virtio-pci, and they still seem to
>>> be the same. I wonder why we never saw this on virtio-pci?
>>
>> Just because we never ran the right tests, probably.  The bug is indeed
>> in virtio-pci as well (I didn't check mmio).
> 
> Somebody should probably do a writeup on the expected behaviour, as
> this always ends in mental gymnastics (at least for me :)

Yeah, it doesn't help that the functions are underdocumented (as in the
"assign" parameter above).

>>
>>>> In dataplane, instead, all calls to
>>>> virtio_queue_set_host_notifier_fd_handler and
>>>> virtio_queue_aio_set_host_notifier_handler should have assign=true.  The
>>>> ioeventfd is just being moved from one aiocontext to another.
>>>
>>> How can a transport know where they are called from?
>>
>> That's a bug in dataplane.  The calls should be changed in
>> hw/block/dataplane/virtio-blk.c
> 
> I don't think the ->set_host_notifiers() api really allows for this.

I think it does, assign is the last argument to k->set_host_notifier().

Paolo