Re: [Qemu-devel] [PATCH for-2.10 0/3] qdev/vfio: defer DEVICE_DEL to avoid races with libvirt

[Michael Roth]

Quoting Michael Roth (2017-07-26 20:30:52)
> This series was motivated by the discussion in this thread:
> 
>   https://www.redhat.com/archives/libvir-list/2017-June/msg01370.html
> 
> The issue this series addresses is that when libvirt unplugs a VFIO PCI 
> device,
> it may attempt to bind the host device back to the host driver when QEMU emits
> the DEVICE_DELETED event for the corresponding vfio-pci device. However, the
> VFIO group FD is not actually cleaned up until vfio-pci device is *finalized*
> by QEMU, whereas the event is emitted earlier during device_unparent.
> Depending on the host device and how long certain operations like resetting 
> the
> device might take, this can in result in libvirt trying to rebind the device
> back to the host while it is still in use by VFIO, leading to host crashes or
> other unexpected behavior.
> 
> In particular, Mellanox CX4 adapters on PowerNV hosts might not be fully
> quiesced by vfio-pci's finalize() routine until up to 6s after the
> DEVICE_DELETED was emitted, leading to detach-device on the libvirt side 
> pretty
> much always crashing the host.
> 
> Implementing this change requires 2 prereqs to ensure the same information is
> available when the DEVICE_DELETED is finally emitted:
> 
> 1) Storing the path in the composition patch, which is addressed by PATCH 1,
>    which was plucked from another pending series from Greg Kurz:
> 
>    https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg07922.html
> 
>    since we are now "disconnected" at the time the event is emitted, and
> 
> 2) Deferring qemu_opts_del of the DeviceState->QemuOpts till finalize, since
>    that is where DeviceState->id is stored. This was actually how it was
>    done in the past, so PATCH 2 simply reverts the change which moved it to
>    device_unparent.
> 
> From there it's just a mechanical move of the event from device_unparent to
> device_finalize.

Ping.

The situation has changed somewhat since original posting as Alex now
has a fix on the kernel side for the VFIO issue noted above:

  
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6586b561a91cd80a91c8f107ed0d144feb3eadc2

However, I think this series would still be useful for addressing the
issue for hosts using older kernels, and there seems to be general
interest from the libvirt side in aligning "DEVICE_DELETED" events
with the notion that QEMU is completely finished with a device.

Patch 1/3 is also needed for another series that Greg is working on
and I don't want to hold that up for this series, so if it's
preferred that we just post that patch separately in the meantime
please let me know.

Thanks!

> 
>  hw/core/qdev.c         | 30 +++++++++++++++++++-----------
>  include/hw/qdev-core.h |  1 +
>  2 files changed, 20 insertions(+), 11 deletions(-)
> 
>