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

[Ryan Harper <ryanh@us.ibm.com>]

* Markus Armbruster <armbru@redhat.com> [2010-11-05 11:11]:
> Ryan Harper <ryanh@us.ibm.com> writes:
> 
> > * Markus Armbruster <armbru@redhat.com> [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.


-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com