Re: [Qemu-devel] [RFC PATCH v4 09/11] spapr: Support topologies with unfilled cores

From: Bharata B Rao <bharata@linux.vnet.ibm.com>
To: Thomas Huth <thuth@redhat.com>
Cc: mdroth@linux.vnet.ibm.com, aik@ozlabs.ru, agraf@suse.de,
	qemu-devel@nongnu.org, qemu-ppc@nongnu.org,
	tyreld@linux.vnet.ibm.com, imammedo@redhat.com,
	nfont@linux.vnet.ibm.com, afaerber@suse.de,
	David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [Qemu-devel] [RFC PATCH v4 09/11] spapr: Support topologies with unfilled cores
Date: Wed, 9 Sep 2015 12:28:02 +0530	[thread overview]
Message-ID: <20150909065802.GH17433@in.ibm.com> (raw)
In-Reply-To: <55E95A09.8030700@redhat.com>

On Fri, Sep 04, 2015 at 10:44:57AM +0200, Thomas Huth wrote:
> On 04/09/15 09:01, David Gibson wrote:
> > On Thu, Aug 06, 2015 at 10:57:15AM +0530, Bharata B Rao wrote:
> >> QEMU currently supports CPU topologies where there can be cores
> >> which are not completely filled with all the threads as per the
> >> specifed SMT mode.
> >>
> >> Restore support for such topologies (Eg -smp 15,cores=4,threads=4)
> >> The last core will always have the deficit even when -device options are
> >> used to cold-plug the cores.
> >>
> >> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > 
> > Is there a reason to support these silly toplogies, or should we just
> > error out if this is specified?

Only reason was to ensure that existing guest with such topologies
continue to boot like before.

> 
> FYI, I've recently submitted a patch that tries to catch such illegal
> SMP configurations and simply errors out in that case:
> 
> http://lists.nongnu.org/archive/html/qemu-devel/2015-07/msg04549.html
> 
> It's not upstream yet, but already in Eduardo's x86 branch. I think this
> will reject the bad topology from your example, too.

It does reject -smp 15,cores=4,threads=4, but with

-smp 15,cores=4,threads=4,maxcpus=16, the guest still boots with weird
topology.

[root@localhost ~]# lscpu
Architecture:          ppc64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Big Endian
CPU(s):                16
On-line CPU(s) list:   0-14
Off-line CPU(s) list:  15
Thread(s) per core:    3
Core(s) per socket:    1
Socket(s):             4
NUMA node(s):          1
Model:                 IBM pSeries (emulated by qemu)
L1d cache:             64K
L1i cache:             32K
NUMA node0 CPU(s):     0-14

[root@localhost ~]# ppc64_cpu --info
Core   0:    0*    1*    2*    3* 
Core   1:    4*    5*    6*    7* 
Core   2:    8*    9*   10*   11* 
Core   3:   12*   13*   14*   15

Should such topologies also be prevented from booting ?

Regards,
Bharata.