Re: [Qemu-devel] [PATCH RFC 06/16] vl: move smp parsing to machine pre_init

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Jun 14, 2016 at 04:03:29PM +0200, Andrew Jones wrote:
> On Tue, Jun 14, 2016 at 01:53:05PM +0200, Paolo Bonzini wrote:
> > 
> > 
> > On 14/06/2016 13:39, Andrew Jones wrote:
> > > On Tue, Jun 14, 2016 at 10:17:49AM +0200, Paolo Bonzini wrote:
> > >> On 13/06/2016 22:35, Andrew Jones wrote:
> > >>> On Mon, Jun 13, 2016 at 07:04:01PM +0200, Paolo Bonzini wrote:
> > >>>> On 10/06/2016 19:40, Andrew Jones wrote:
> > >> They should just not specify it and get a default of 1. ;)
> > > 
> > > Yeah, threads, the only one we should never calculate, could be
> > > optional. If not specified, defaulting to 1 makes perfect sense.
> > > But, threads=0, which is weird, but in a way specifying that it's
> > > not specified, also makes some sense.
> > 
> > If it's weird, let's make it invalid. :)
> 
> I'm weird, and starting to like it :-)
> 
> > 
> > >>>> - cpus % (cores * threads) != 0
> > >>>
> > >>> Hmm. This makes sense where cpus is the number of cpu packages,
> > >>
> > >> I'm not sure I understand what you mean here.  The point is that the
> > >> machine starts with an integral number of sockets.
> > > 
> > > OK, s/cpus/maxcpus/ then. By using the currently online number, I
> > > thought you were starting to prepare for cpu packages, which are
> > > indivisible sets of cores and threads.
> > 
> > Yes, that's what I meant to do.  Isn't cpus what you call
> > "total-online-threads" below?
> 
> It is. I was thinking it may need to be redefined if we wanted to
> hotplug these "indivisible sets of cores and threads" (sockets),
> instead of what we do today, which is to hotplug one "cpu" at a time.
> However, I just chatted with Igor, and he says cpu hotplug operates
> on logical processors, and thus it's fine to talk about hotplugging
> threads. Even real hardware does this. Real hardware will plug a
> socket full of threads, but then the firmware may keep most of them
> disabled until they're "hotplugged". So, I think the value of cpus
> can be anything. Even the useless value of zero.

Uh.. this depends on the platform (machine type).  ACPI allows
(logical) hotplugging of individual threads.  However on pseries the
hotplug mechanism works at core granularity only.  I don't know of
real examples off hand, but it's easy to imagine a platform interface
which worked at socket, or even a multi-socket-module level

In other words the "cpu package" doesn't necessarily lie at a fixed
level of the thread/core/socket heirarchy.

This is something we're grappling with in trying to implement
cross-platform cpu hotplug, and we obviously don't want to make it
worse with these -smp changes.

-- 
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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