Re: [Qemu-devel] [PATCH 0/3] v4 Decouple block device removal from device removal

[Markus Armbruster]

[Note cc: Michael]

Ryan Harper <address@hidden> writes:

> This patch series decouples the detachment of a block device from the removal
> of the backing pci-device.  Removal of a hotplugged pci device requires the
> guest to respond before qemu tears down the block device. In some cases, the
> guest may not respond leaving the guest with continued access to the block
> device.  
>
> The new monitor command, drive_unplug, will revoke a guests access to the
> block device independently of the removal of the pci device.
>
> The first patch adds a new drive find method, the second patch implements the
> monitor command and block layer changes.
>
> Changes since v3:
> - Moved QMP command for drive_unplug() to separate patch
>
> Changes since v2:
> - Added QMP command for drive_unplug()
>
> Changes since v1:
> - CodingStyle fixes
> - Added qemu_aio_flush() to bdrv_unplug()
>
> Signed-off-by: Ryan Harper <address@hidden>

If I understand your patch correctly, the difference between your
drive_unplug and my blockdev_del is as follows:

* drive_unplug forcefully severs the connection between the host part of
  the block device and its BlockDriverState.  A shell of the host part
  remains, to be cleaned up later.  You need forceful disconnect
  operation to be able to revoke access to an image whether the guest
  cooperates or not.  Fair enough.

* blockdev_del deletes a host part.  My current version fails when the
  host part is in use.  I patterned that after netdev_del, which used to
  work that way, until commit 2ffcb18d:

    Make netdev_del delete the netdev even when it's in use
    
    To hot-unplug guest and host part of a network device, you do:
    
        device_del NIC-ID
        netdev_del NETDEV-ID
    
    For PCI devices, device_del merely tells ACPI to unplug the device.
    The device goes away for real only after the guest processed the ACPI
    unplug event.
    
    You have to wait until then (e.g. by polling info pci) before you can
    unplug the netdev.  Not good.
    
    Fix by removing the "in use" check from do_netdev_del().  Deleting a
    netdev while it's in use is safe; packets simply get routed to the bit
    bucket.

  Isn't this the very same problem that's behind your drive_unplug?

I'd like to have some consistency among net, block and char device
commands, i.e. a common set of operations that work the same for all of
them.  Can we agree on such a set?

Even if your drive_unplug shouldn't fit in that set, we might want it as
a stop-gap.  Depends on how urgent the need for it is.  Yet another
special-purpose command to be deprecated later.