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

[Cornelia Huck]

On Wed, 16 Mar 2016 12:59:37 +0100
Paolo Bonzini <address@hidden> wrote:

> On 16/03/2016 12:56, Cornelia Huck wrote:
> > On Wed, 16 Mar 2016 12:48:22 +0100
> > Paolo Bonzini <address@hidden> wrote:
> > 
> >>
> >>
> >> On 16/03/2016 12:32, Cornelia Huck wrote:
> >>> On Wed, 16 Mar 2016 12:09:02 +0100
> >>> Paolo Bonzini <address@hidden> 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).

My understanding is:

- 'assign': set up a new notifier (true) or disable it (false)
- 'set_handler': use our handler (true) or have it handled elsewhere
  (false)

> 
> >>
> >>>> 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().

This depends on whether we want 'assign' to clean up any old notifiers
before setting up new ones. I think we want different behaviour for
dataplane and vhost.