Re: [Qemu-devel] [RFC 0/6] enable numa configuration before machine_init() from HMP/QMP

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Oct 20, 2017 at 12:21:30PM +1100, David Gibson wrote:
> On Thu, Oct 19, 2017 at 02:23:04PM +0200, Paolo Bonzini wrote:
> > On 19/10/2017 13:49, David Gibson wrote:
> > > Note that describing socket/core/thread tuples as arch independent (or
> > > even machine independent) is.. debatable.  I mean it's flexible enough
> > > that most platforms can be fit to that scheme without too much
> > > straining.  But, there's no arch independent way of defining what each
> > > level means in terms of its properties.
> > > 
> > > So, for example, on spapr - being paravirt - there's no real
> > > distinction between cores and sockets, how you divide them up is
> > > completely arbitrary.
> > 
> > Same on x86, actually.
> > 
> > It's _common_ that cores on the same socket share L3 cache and that a
> > socket spans an integer number of NUMA nodes, but it doesn't have to be
> > that way.
> > 
> > QEMU currently enforces the former (if it tells the guest at all that
> > there is an L3 cache), but not the latter.
> 
> Ok.  Correct me if I'm wrong, but doesn't ACPI describe the NUMA
> architecture in terms of this thread/core/socket heirarchy?  That's
> not true for PAPR, where the NUMA topology is described in an
> independent set of (potentially arbitrarily nested) nodes.

On PC, ACPI NUMA information only refer to CPU APIC IDs, which
identify individual CPU threads; it doesn't care about CPU
socket/core/thread topology.  If I'm not mistaken, the
socket/core/thread topology is not represented in ACPI at all.

Some guest OSes, however, may get very confused if they see an
unexpected NUMA/CPU topology.  IIRC, it was possible to make old
Linux kernel versions panic by generating a weird topology.

-- 
Eduardo