Re: [Qemu-devel] [RFC PATCH v0] spapr: Disable CPU unplug in TCG mode

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

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On Wed, Sep 21, 2016 at 10:18:00AM +0530, Bharata B Rao wrote:
> CPU unplug doesn't work in TCG mode currently and causes frequent system
> freeze. In addition to other potential problems, the main problem arises
> of out the requirement to support synchronous removal of a CPU thread.
> The CPU thread that performs the cleanup of the unplugged CPU, kicks and
> waits for the unplugged CPU thread to finish. This wait never finishes in
> TCG mode when the waiting thread and the unplugged CPU thread are one and
> the same.
> 
> So wait till proper MTTCG support is available before enabling
> CPU unplug in TCG mode.

MTTCG seems like a very big hammer to fix this with.  Surely we could
come up with a simpler interlock that would work for TCG in the
meantime.

> 
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> ---
>  hw/ppc/spapr.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index ca77bb0..d7864c6 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2300,6 +2300,10 @@ static void spapr_machine_device_unplug(HotplugHandler *hotplug_dev,
>              error_setg(errp, "CPU hot unplug not supported on this machine");
>              return;
>          }
> +        if (!kvm_enabled()) {
> +            error_setg(errp, "CPU hot unplug not supported in TCG mode");
> +            return;
> +        }
>          spapr_core_unplug(hotplug_dev, dev, errp);
>      }
>  }

-- 
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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