Re: [Qemu-devel] [Qemu-ppc] [PATCH] pseries: do not allow memory-less/cpu-less NUMA node

[David Gibson]

On Fri, Aug 30, 2019 at 07:45:43PM +0200, Greg Kurz wrote:
> On Fri, 30 Aug 2019 17:34:13 +0100
> Daniel P. Berrangé <address@hidden> wrote:
> 
> > On Fri, Aug 30, 2019 at 06:13:45PM +0200, Laurent Vivier wrote:
> > > When we hotplug a CPU on memory-less/cpu-less node, the linux kernel
> > > crashes.
> > > 
> > > This happens because linux kernel needs to know the NUMA topology at
> > > start to be able to initialize the distance lookup table.
> > > 
> > > On pseries, the topology is provided by the firmware via the existing
> > > CPUs and memory information. Thus a node without memory and CPU cannot be
> > > discovered by the kernel.
> > > 
> > > To avoid the kernel crash, do not allow to start pseries with empty
> > > nodes.
> > 
> > This describes one possible guest OS. Is there any reasonable chance
> > that a non-Linux guest might be able to handle this situation correctly,
> > or do you expect any guest to have the same restriction ?

That's... a more complicated question than you'd think.

The problem here is it's not really obvious in PAPR how topology
information for nodes without memory should be described in the device
tree (which is the only way we given that information to the guest).

It's possible there's some way to encode this information that would
make AIX happy and we just need to fix Linux to cope with that, but
it's not really clear what it would be.

> I can try to grab an AIX image and give a try, but anyway this looks like
> a very big hammer to me... :-\

I'm not really sure why everyone seems to think losing zero-memory
node capability is such a big deal.  It's never worked in practice on
POWER and we can always put it back if we figure out a sensible way to
do it.

-- 
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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