Re: [Qemu-devel] [Qemu-ppc] [PATCH v2 0/2] spapr: disable hotplugging without OS

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

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On Thu, Jun 08, 2017 at 03:35:58PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 06/08/2017 02:27 PM, Laurent Vivier wrote:
> > If the OS is not started, QEMU sends an event to the OS
> > that is lost and cannot be recovered. An unplug is not
> > able to restore QEMU in a coherent state.
> > So, while the OS is not started, disable CPU and memory hotplug.
> > We guess the OS is started if the CAS has been negotiated.
> > 
> > This series also revert previous fix which was not really fixing
> > the hotplug problem when the OS is not running.
> > 
> > v2:
> > - fix indent
> > - remove useless local_err
> > - allow hotplug if the VM is not started (pre-launch or incoming state)
> > - remove vector 6, instead just mark the end of CAS negotiation
> > 
> > Laurent Vivier (2):
> >    spapr: disable hotplugging without OS
> >    Revert "spapr: fix memory hot-unplugging"
> > 
> >   hw/ppc/spapr.c             | 51 +++++++++++++++++++++++++++++++++++++++++++---
> >   hw/ppc/spapr_drc.c         | 20 +++---------------
> >   hw/ppc/spapr_hcall.c       |  1 +
> >   include/hw/ppc/spapr.h     |  2 ++
> >   include/hw/ppc/spapr_drc.h |  1 -
> >   5 files changed, 54 insertions(+), 21 deletions(-)
> > 
> Tested-by: Daniel Barboza <danielhb@linux.vnet.ibm.com>
> 
> This is curious. I was having a little look into hotplug/unplug and DRC
> states yesterday
> while taking a look at the latest David's cleanup. I was trying to find out
> a way to tune
> spapr_drc_needed() in a way that we don't accidentally migrate more DRCs
> than necessary
> and at the same time handle that scenario of libvirt migration after hotplug
> (libvirt hotplugs
> the device on target instead of adding it in the command line).
> 
> I would like to migrate the DRCs if and only if:
> 
> 1 - a device was hotplugged
> 2 - a device is undergoing hotplug/unplug
> 
> (2) is easy and is already taken care of. But (1) is tricky because, before
> this patch, if a migration
> occurs *before* CAS we don't need to migrate the DRC - the object will go
> through the state
> changes after the migration. With this series this scenario is not going to
> happen, then
> we can happily add a
> 
> if (dev->hotplugged) return true;
> 
> in spapr_drc_needed and fix the libvirt migration scenario again.

As mentioned in another thread, I'm looking at explicitly resetting
all DRC states during the CAS process.  I think that will simplify
these cases.

-- 
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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