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

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

* Michael S. Tsirkin <mst@redhat.com> [2010-11-03 02:22]:
> On Tue, Nov 02, 2010 at 03:23:38PM -0500, Ryan Harper wrote:
> > * Michael S. Tsirkin <mst@redhat.com> [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)
> 
> Yes. Long term I think we will want a way to do that.
> 
> > 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
> 
> This is what drive_unplug does, right?

Correct.

> 
> > 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.
> 
> By response you mean eject?  No, this is not what I was suggesting.
> I was really suggesting that your patch is fine :)
> Sorry about confusion.

I don't mean eject; I mean responding to the ACPI event by writing a
response to the PCI chipset which QEMU then in turn will invoke the
qdev_unplug() path which ultimately kills the device and the Drive and
BlockState objects.

> 
> I was also saying that from what I hear, the pci express support
> will at some point need interfaces to
> - notify guest about device removal/addition
> - get eject from guest
> - remove device without talking to guest
> - add device without talking to guest
> - suppress device deletion on eject
> 
> All this can be generic and can work through express
> configuration mechanisms or through acpi for pci.
> But this is completely separate from unplugging
> the host backend, which should be possible at any point.

Yes.  I think we've worked out that we do want an independent
unplug/disconnect mechanism rather than tying it to device_del.

Marcus, it sounds like then you wanted to see a net_unplug/disconnect
and that instead of having device_del always succeed and replacing it
with a shell, we'd need to provide an explicit command to do the
disconnect in a similar fashion to how we're doing drive_unplug?

With at least two of these device types needing an explicit disconnect
to sever the bond between host/guest makes me want a device-level
interface for doing the disconnect that each device can implement
differently.



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