Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc/pnv: fix cores per chip for multiple cpus

[Greg Kurz <groug@kaod.org>]

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On Fri, 22 Sep 2017 21:20:42 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Sep 22, 2017 at 12:46:58PM +0200, Cédric Le Goater wrote:
> > On 09/22/2017 12:12 PM, David Gibson wrote:  
> > > On Fri, Sep 22, 2017 at 08:07:06AM +0200, Cédric Le Goater wrote:  
> > >> On 09/22/2017 08:00 AM, Nikunj A Dadhania wrote:  
> > >>> David Gibson <david@gibson.dropbear.id.au> writes:
> > >>>  
> > >>>>>>>
> > >>>>>>> As smp_thread defaults to 1 in vl.c, similarly smp_cores also has the
> > >>>>>>> default value of 1 in vl.c. In powernv, we were setting nr-cores like
> > >>>>>>> this:
> > >>>>>>>
> > >>>>>>>         object_property_set_int(chip, smp_cores, "nr-cores", &error_fatal);
> > >>>>>>>
> > >>>>>>> Even when there were multiple cpus (-smp 4), when the guest boots up, we
> > >>>>>>> just get one core (i.e. smp_cores was 1) with single thread(smp_threads
> > >>>>>>> was 1), which is wrong as per the command-line that was provided.  
> > >>>>>>
> > >>>>>> Right, so, -smp 4 defaults to 4 sockets, each with 1 core of 1
> > >>>>>> thread.  If you can't supply 4 sockets you should error, but you
> > >>>>>> shouldn't go and change the number of cores per socket.  
> > >>>>>
> > >>>>> OK, that makes sense now. And I do see that smp_cpus is 4 in the above
> > >>>>> case. Now looking more into it, i see that powernv has something called
> > >>>>> "num_chips", isnt this same as sockets ? Do we need num_chips separately?  
> > >>>>
> > >>>> Ah, yes, I see.  It's probably still reasonable to keep num_chips as
> > >>>> an internal variable, rather than using (smp_cpus / smp_cores /
> > >>>> smp_threads) everywhere.  But we shouldn't have it as a direct
> > >>>> user-settable property, instead setting it from the -smp command line
> > >>>> option.  
> > >>>
> > >>> Something like the below works till num_chips=2, after that guest does
> > >>> not boot up. This might be some limitation within the OS, Cedric might
> > >>> have some clue.  
> > >>
> > >> Some controllers might need some more tweaking, PSI, LPC, to elect a 
> > >> master one.  
> > > 
> > > Uh.. why?  
> > 
> > that's not true. I managed to boot a pnv machine with 4 chips/sockets 
> > each having 4 cpus using a 4.4.9-openpower2 skiroot kernel, from an 
> > openpower firmare 1.10 I think. Recent openpower kernel must be using 
> > some new features/instructions that we don't manage well in QEMU.
> > 
> > I would need to build a kernel with more output.
> >   
> > >> Anyhow I don't think we want to deduce the number of chips
> > >> from the number of cpus. These two figures are very different.  
> > > 
> > > How so?  It's not totally perfect, but making a single chip correspond
> > > to a "socket" in qemu's (somewhat x86 centric) terminology is still a
> > > pretty good match.  
> > 
> > well, it would be good to be able to define chips with different
> > numbers of cpus. That is something will we want to do for sure.  
> 
> You mean multiple chips in a single system with non-uniform numbers of
> cores?  Are there really such systems in the wild?
> 

Doesn't it happen when a CPU core gets deconfigured ?

-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/LTC                      http://www.ibm.com
Tel 33-5-6218-1607

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.

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