Re: [Qemu-devel] [PATCH] spapr: manage hotplugged devices while the VM is not started

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jun 08, 2017 at 03:00:53PM -0500, Michael Roth wrote:
> Quoting David Gibson (2017-05-30 23:35:57)
> > On Tue, May 30, 2017 at 06:04:45PM +0200, Laurent Vivier wrote:
> > > For QEMU, a hotlugged device is a device added using the HMP/QMP
> > > interface.
> > > For SPAPR, a hotplugged device is a device added while the
> > > machine is running. In this case QEMU doesn't update internal
> > > state but relies on the OS for this part
> > > 
> > > In the case of migration, when we (libvirt) hotplug a device
> > > on the source guest, we (libvirt) generally hotplug the same
> > > device on the destination guest. But in this case, the machine
> > > is stopped (RUN_STATE_INMIGRATE) and QEMU must not expect
> > > the OS will manage it as an hotplugged device as it will
> > > be "imported" by the migration.
> > > 
> > > This patch changes the meaning of "hotplugged" in spapr.c
> > > to manage a QEMU hotplugged device like a "coldplugged" one
> > > when the machine is awaiting an incoming migration.
> > > 
> > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > 
> > So, I think this is a reasonable concept, at least in terms of
> > cleanliness and not doing unnecessary work.  However, if it's fixing
> > bugs, I suspect that means we still have problems elsewhere.
> 
> I was hoping a lot of these issues would go away once we default
> the initial/reset DRC states to "coldplugged". I think your pending
> patch:
> 
>   "spapr: Make DRC reset force DRC into known state"
> 
> But I didn't consider the fact that libvirt will be issuing these
> hotplugs *after* reset, so those states would indeed need to
> be fixed up again to reflect boot-time,attached as opposed to
> boot-time,unattached before starting the target.

So, something I haven't written yet, but I'm considering for my DRC
cleanup series is to force a reset of the DRC objects (and just the
DRC objects) at CAS time.  Since that's when the guest gets the final
version of the device tree, that should ensure that the DRC state is
in sync with whether the guest sees the device as hotplugged or
coldplugged.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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