Re: [Qemu-ppc] [PATCH 1/1] spapr: Ensure CPU cores are added contiguously and removed in LIFO order

[Igor Mammedov]

On Wed, 13 Jul 2016 12:20:20 +0530
Bharata B Rao <address@hidden> wrote:

> If CPU core addition or removal is allowed in random order leading to
> holes in the core id range (and hence in the cpu_index range), migration
> can fail as migration with holes in cpu_index range isn't yet handled
> correctly.
> 
> Prevent this situation by enforcing the addition in contiguous order
> and removal in LIFO order so that we never end up with holes in
> cpu_index range.
Adding this limitation looks better than adding migration_id as
it will allow libvirt to use the current -numa cpus=... while
doing the same amount of guess work as it does now.

Similar patch for x86 won't be so simple as cpu-add can add cpus
with gaps (breaking migration at that), so I'd need to keep it
that way with some compat code, but that shouldn't be issue.


> Signed-off-by: Bharata B Rao <address@hidden>
> ---
> While there is work in progress to support migration when there are holes
> in cpu_index range resulting from out-of-order plug or unplug, this patch
> is intended as a last resort if no easy, risk-free and elegant solution
> emerges before 2.7 dev cycle ends.
> 
>  hw/ppc/spapr_cpu_core.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> index bc52b3c..4bfc96b 100644
> --- a/hw/ppc/spapr_cpu_core.c
> +++ b/hw/ppc/spapr_cpu_core.c
> @@ -126,12 +126,23 @@ static void spapr_core_release(DeviceState *dev, void 
> *opaque)
>  void spapr_core_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
>                         Error **errp)
>  {
> +    sPAPRMachineState *spapr = SPAPR_MACHINE(OBJECT(hotplug_dev));
>      CPUCore *cc = CPU_CORE(dev);
>      sPAPRDRConnector *drc =
>          spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_CPU, cc->core_id);
>      sPAPRDRConnectorClass *drck;
>      Error *local_err = NULL;
> +    int smt = kvmppc_smt_threads();
> +    int index = cc->core_id / smt;
> +    int spapr_max_cores = max_cpus / smp_threads;
> +    int i;
>  
> +    for (i = spapr_max_cores - 1; i > index; i--) {
> +        if (spapr->cores[i]) {
> +            error_setg(errp, "core-id %d should be removed first", i * smt);
> +            return;
> +        }
> +    }
>      g_assert(drc);
>  
>      drck = SPAPR_DR_CONNECTOR_GET_CLASS(drc);
> @@ -214,7 +225,7 @@ void spapr_core_pre_plug(HotplugHandler *hotplug_dev, 
> DeviceState *dev,
>      sPAPRMachineClass *smc = SPAPR_MACHINE_GET_CLASS(OBJECT(hotplug_dev));
>      sPAPRMachineState *spapr = SPAPR_MACHINE(OBJECT(hotplug_dev));
>      int spapr_max_cores = max_cpus / smp_threads;
> -    int index;
> +    int index, i;
>      int smt = kvmppc_smt_threads();
>      Error *local_err = NULL;
>      CPUCore *cc = CPU_CORE(dev);
> @@ -252,6 +263,14 @@ void spapr_core_pre_plug(HotplugHandler *hotplug_dev, 
> DeviceState *dev,
>          goto out;
>      }
>  
> +    for (i = 0; i < index; i++) {
> +        if (!spapr->cores[i]) {
> +            error_setg(&local_err, "core-id %d should be added first",
> +                       i * smt);
> +            goto out;
> +        }
> +    }
> +
>  out:
>      g_free(base_core_type);
>      error_propagate(errp, local_err);