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

[Ryan Harper]

* 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?

> 
> 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.

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.

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
address@hidden