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

[David Gibson]

On Mon, Sep 02, 2019 at 09:57:36AM +0100, Daniel P. Berrangé wrote:
> On Mon, Sep 02, 2019 at 04:27:18PM +1000, David Gibson wrote:
> > 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.
> 
> I'm not that bothered - I just wanted to double check that we were not
> intentionally breaking a non-Linux guest OS that was known to work today.

There are no non-Linux guests that are known to work today, unless you
count the kvm-unit-tests micro-OS.  AIX support is coming along, but
it's by no means established.

-- 
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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