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


From: Cornelia Huck
Subject: Re: [Qemu-block] [PATCH 0/4] Tweaks around virtio-blk start/stop
Date: Wed, 16 Mar 2016 12:56:23 +0100

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 :)

> 
> >> 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.




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