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

[Markus Armbruster]

Ryan Harper <address@hidden> writes:

> * Markus Armbruster <address@hidden> [2010-11-06 04:19]:
>> Ryan Harper <address@hidden> writes:
>> 
>> > * Markus Armbruster <address@hidden> [2010-11-05 11:11]:
>> >> Ryan Harper <address@hidden> writes:
>> >> 
>> >> > * Markus Armbruster <address@hidden> [2010-11-05 08:28]:
>> >> >> I'd be fine with any of these:
>> >> >> 
>> >> >> 1. A new command "device_disconnet ID" (or similar name) to disconnect
>> >> >>    device ID from any host parts.  Nice touch: you don't have to know
>> >> >>    about the device's host part(s) to disconnect it.  But it might be
>> >> >>    more work than the other two.
>> >> >
>> >> > This is sort of what netdev_del() and drive_unplug() are today; we're
>> >> > just saying sever the connection of this device id.   
>> >> 
>> >> No, I have netdev_del as (3).
>> >> 
>> >> All three options are "sort of" the same, just different commands with
>> >> a common purpose.
>> >> 
>> >> > I'd like to rename drive_unplug() to blockdev_del() and call it done.  I
>> >> > was looking at libvirt and the right call to netdev_del is already
>> >> > in-place; I'd just need to re-spin my block patch to call blockdev_del()
>> >> > after invoking device_del() to match what is done for net.
>> >> 
>> >> Unless I'm missing something, you can't just rename: your unplug does
>> >> not delete the host part.
>> >> 
>> >> >> 2. New commands netdev_disconnect, drive_disconnect (or similar names)
>> >> >>    to disconnect a host part from a guest device.  Like (1), except you
>> >> >>    have to point to the other end of the connection to cut it.
>> >> >
>> >> > What's the advantage here? We need an additional piece of info (host
>> >> > part) in addition to the device id?
>> >> 
>> >> That's a disadvantage.
>> >> 
>> >> Possible advantage: implementation could be slightly easier than (1),
>> >> because you don't have to find the host parts.
>> >> 
>> >> >> 3. A new command "drive_del ID" similar to existing netdev_del.  This 
>> >> >> is
>> >> >>    (2) fused with delete.  Conceptual wart: you can't disconnect and
>> >> >>    keep the host part around.  Moreover, delete is slightly dangerous,
>> >> >>    because it renders any guest device still using the host part
>> >> >>    useless.
>> >> >
>> >> > Hrm, I thought that's what (1) is.
>> >> 
>> >> No.
>> >> 
>> >> With (1), the argument is a *device* ID, and we disconnect *all* host
>> >> parts connected to this device (typically just one).
>> >> 
>> >> With (3), the argument is a netdev/drive ID, and disconnect *this* host
>> >> part from the peer device.
>> >> 
>> >> >                                     Well, either (1) or (3); I'd like to
>> >> > rename drive_unplug() to blockdev_del() since they're similar function
>> >> > w.r.t removing access to the host resource.  And we can invoke them in
>> >> > the same way from libvirt (after doing guest notification, remove
>> >> > access).
>> >> 
>> >> I'd call it drive_del for now, to match drive_add.
>> >
>> > OK, drive_del() and as you mentioned, drive_unplug will take out the
>> > block driver, but doesn't remove the dinfo object; that ends up dying
>> > when we call the device destructor.  I think for symmetry we'll want
>> > drive_del to remove the dinfo object as well.
>> 
>> Exactly.
>> 
>> a. bdrv_detach() to zap the pointer from bdrv to qdev
>> b. zap the pointer from qdev to bdrv
>> c. drive_uninit() to dispose of the host part
>
> a-c need to be done to match netdev_del symmetry?  How hard of a req is
> this?

Without (c), it's not a delete.  And (c) without (b) leaves a dangling
pointer.  (c) without (a) fails an assertion in bdrv_delete().

Aside: (b) should probably be folded into bdrv_detach().

>> Step b could be awkward with (3), because you don't know device details.
>> I guess you have to search device properties for a drive property
>> pointing to bdrv.  I like (1) because it puts that loop in the one place
>> where it belongs: qdev core.  (3) duplicates it in every HOSTDEV_del.
>> Except for netdev_del, which is special because of VLANs.
>> 
>> To avoid step b, you could try to keep the bdrv around in a special
>> zombie state.  Still have to free the dinfo, but can't use
>> drive_uninit() for that then.
>> 
>> If you think I'm overcomplicating this, feel free to prove me wrong with
>> working code :)
>
> drive_unplug() works as-is today; so it does feel very combursome at
> this point.  Other than the name change and agreement on how mgmt should
> invoke the command, it's been a long ride to get here.

Sometimes it takes a tough man to make a tender chicken.

> I'll take my best shot at trying to clean up the other
> pointers and objects; though on one of my attempts when I took out the
> dinfo() object that didn't go so well; going to have to audit who uses
> dinfo and where and what they check before calling it to have a proper
> cleanup that doesn't remove the whole device altogether.

Steps a, b, c are the result of my (admittedly quick) audit.

Here's how the various objects are connected to each other:

               contains
drivelist    -----------> DriveInfo
                                |
                                | .bdrv
                                | .id == .bdrv->device_name
                                |
               contains         V
bdrv_states  -----------> BlockDriverState
                             |   ^
                       .peer |   |
                             |   |                          host part
-----------------------------|---|-----------------------------------
                             |   |                         guest part
                             |   | property "drive"
                             v   |
                          DeviceState

To disconnect host from guest part, you need to cut both pointers.  To
delete the host part, you need to delete both objects, BlockDriverState
and DriveInfo.