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

[Daniel P . Berrangé]

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.

Regards,
Daniel
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