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

[Ryan Harper]

* Michael S. Tsirkin <address@hidden> [2010-11-02 14:18]:
> On Tue, Nov 02, 2010 at 02:01:08PM -0500, Ryan Harper wrote:
> > > > > > I like the idea of disconnect; if part of the device_del method was 
> > > > > > to
> > > > > > invoke a disconnect method, we could implement that for block, net, 
> > > > > > etc;
> > > > > > 
> > > > > > I'd think we'd want to send the notification, then disconnect.
> > > > > > Struggling with whether it's worth having some reasonable timeout
> > > > > > between notification and disconnect.  
> > > > > 
> > > > > The problem with this is that it has no analog in real world.
> > > > > In real world, you can send some notifications to the guest, and you 
> > > > > can
> > > > > remove the card.  Tying them together is what created the problem in 
> > > > > the
> > > > > first place.
> > > > > 
> > > > > Timeouts can be implemented by management, maybe with a nice dialog
> > > > > being shown to the user.
> > > > 
> > > > Very true.  I'm fine with forcing a disconnect during the removal path
> > > > prior to notification.  Do we want a new disconnect method at the device
> > > > level (pci)? or just use the existing removal callback and call that
> > > > during the initial hotremov event?
> > > 
> > > Not sure what you mean by that, but I don't see a device doing anything
> > > differently wrt surprise or ordered removal. So probably the existing
> > > callback should do. I don't think we need to talk about disconnect:
> > > since we decided we are emulating device removal, let's call it
> > > just that.
> > 
> > Because current the "removal" process depends on the guest actually
> > responding.  What I'm suggesting is that, in Marcus's term, and what
> > drive_unplug() implements, is to disconnect the host block device from
> > the guest device to prevent any further access to it in the case the
> > guest doesn't respond to the removal request made via ACPI.
> > 
> > Very specifically, what we're suggesting instead of the drive_unplug()
> > command so to complete the device removal operation without waiting for
> > the guest to respond; that's what's going to happen if we invoke the
> > response callback; it will appear as if the guest responded whether it
> > did or not.
> > 
> > What I was suggesting above was to instead of calling the callback for
> > handing the guest response was to add a device function called
> > disconnect which would remove any association of host resources from
> > guest resources before we notified the guest.  Thinking about it again
> > I'm not sure this is useful, but if we're going to remove the device
> > without the guests knowledge, I'm not sure how useful sending the
> > removal requests via ACPI is in the first place.
> > 
> > My feeling is that I'd like to have explicit control over the disconnect
> > from host resources separate from the device removal *if* we're going to
> > retain the guest notification.  If we don't care to notify the guest,
> > then we can just do device removal without notifying the guest
> > and be done with it.
> 
> I imagine management would typically want to do this:
> 1. notify guest
> 2. wait a bit
> 3. remove device

Yes; but this argues for (1) being a separate command from (3) unless we
require (3) to include (1) and (2) in the qemu implementation.

Currently we implement:

1. device_del (attempt to remove device)
2. notify guest
3. if guest responds, remove device
4. disconnect host resource from device on destruction

With my drive_unplug patch we do:

1. disconnect host resource from device
2. device_del (attempt to remove device)
3. notify guest
4. if guest responds, remove device

I think we're suggesting to instead do (if we keep disconnect as part of
device_del)

1. device_del (attemp to remove device)
2. notify guest
3. invoke device destruction callback resulting in disconnect host resource 
from device
4. if guest responds, invoke device destruction path a second time.



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