Re: [Qemu-devel] [PATCH] pseries: Restore support for total vcpus not a multiple of threads-per-core for old machine types

[Greg Kurz <groug@kaod.org>]

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On Tue, 23 May 2017 16:36:45 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> As of pseries-2.7 and later, we require the total number of guest vcpus to
> be a multiple of the threads-per-core.  pseries-2.6 and earlier machine
> types, however, are supposed to allow this for the sake of migration from
> old qemu versions which allowed this.
> 
> Unfortunately, 8149e29 "pseries: Enforce homogeneous threads-per-core"
> broke this by not considering the old machine type case.  This fixes it by
> only applying the check when the machine type supports hotpluggable cpus.
> By not-entirely-coincidence, that corresponds to the same time when we
> started enforcing total threads being a multiple of threads-per-core.
> 
> Fixes: 8149e2992f7811355cc34721b79d69d1a3a667dd
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

and

Tested-by: Greg Kurz <groug@kaod.org>

>  hw/ppc/spapr.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index c92d269..bcb0e18 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2863,7 +2863,13 @@ static void spapr_core_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
>          goto out;
>      }
>  
> -    if (cc->nr_threads != smp_threads) {
> +    /*
> +     * In general we should have homogeneous threads-per-core, but old
> +     * (pre hotplug support) machine types allow the last core to have
> +     * reduced threads as a compatibility hack for when we allowed
> +     * total vcpus not a multiple of threads-per-core.
> +     */
> +    if (mc->has_hotpluggable_cpus && (cc->nr_threads != smp_threads)) {
>          error_setg(errp, "invalid nr-threads %d, must be %d",
>                     cc->nr_threads, smp_threads);
>          return;


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