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Re: Availability of physical devices and migration (was: Re: [RFC PATCH

From: Halil Pasic
Subject: Re: Availability of physical devices and migration (was: Re: [RFC PATCH 2/2] vfio-ccw: Connect the device request notifier)
Date: Fri, 20 Nov 2020 13:29:12 +0100

On Fri, 20 Nov 2020 12:38:37 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Fri, 20 Nov 2020 03:51:07 +0100
> Halil Pasic <pasic@linux.ibm.com> wrote:
> > On Tue, 17 Nov 2020 04:26:05 +0100
> > Eric Farman <farman@linux.ibm.com> wrote:
> > 
> > > Now that the vfio-ccw code has a notifier interface to request that
> > > a device be unplugged, let's wire that together.  
> > 
> > I'm aware of the fact that performing an unplug is what vfio-pci does,
> > but I was not aware of this before, and I became curious with regards
> > to how is this going to mesh with migration (in the future).
> > 
> > The sentence 'For this to work, QEMU has to be launched with the same
> > arguments the two times.' form docs/devel/migration.rst should not
> > be taken literally, I know, but the VM configuration changing not because
> > it was changed via a management interface, but because of events that
> > may not be under the control of whoever is managing the VM does
> > make thinks harder to reason about.
> > 
> > I suppose, we now basically mandate that whoever manages the VM, either
> > a) maintains his own model of the VM and listens to the events, to
> > update it if such a device removal happens, or
> > b) inspects the VM at some appropriate point in time, to figure out how
> > the target VM is supposed to be started.
> > 
> > I think libvirt does a).
> This seems like something that would be of general interest to libvirt
> folks, adding the list on cc:.

I agree. Just didn't now if this stuff was already discussed and
worked out.

> For virtual devices, QEMU and any management software are in full
> control, and can simply make sure that both source and target have the
> device available.
> For physical devices, we still can make sure that source and target
> match when we do the setup, but device failures can happen at
> inconvenient times. It may suddenly be no longer possible to access
> state etc. 

Regarding virtual vs physical, I gave this some thought yesterday after
I've sent my previous email. Now I'm not sure virtual devicess and
pass-through devices are all that different. I mean if for example
you have a virtio-blk device that use a SCSI disk device special file
as backing and that SCSI disk (a physical device) fails, the virtio-blk
device is pretty much has going go dysfunctional. And it ain't different,
if it's backed by a file on a filesystem, and the filesystem fails.

I'm not sure I understand the reason why do we hot unplug the qemu
device, in case of pass-through.

I mean if I plug a physical device, into my physical machine, yes it may
fail at any time and even go completely silent (analogous to guest view),
but the physical machine is AFAIK not likely to catapult out the faulty
component (analogous to the QOM entity). At least for the machines I
owned, I always had to grab a screwdriver and unplug the device myself.
What I'm trying to say, this auto-unplug is a little non-intuitive
for me, but it ain't too bad.

> Can we propagate changes like "device foobar, don't bother
> migrating" even when we already started migration? Should the handling
> be different if the target system uses a different (compatible) device?
> Should we fail the migration?

Thanks for asking these questions. I guess, I wanted to ask them myself,
but I'm not very eloquent. I have no answers.

> > 
> > I also suppose, such a device removal may not happen during device
> > migration. That is, form the QEMU perspective I  believe taken care
> > by the BQL.
> Even if the device is not actually removed, it might still be
> inaccessible.
> > 
> > But I'm in the dark regarding the management software/libivrt view. For
> > example what happens if we get a req_notification on the target while
> > pre-copy memory migration? At that point the destination is already
> > receiving pages, thus it is already constructed.
> > 
> > My questions are not necessarily addressed to you Eric. Maybe the
> > folks working on vfio migration can help us out. I'm also adding
> > our libvirt guys.
> > 
> > Regards,
> > Halil
> > 

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