Re: [Qemu-devel] [PATCH] spapr: Set compat mode in spapr_core_plug()

[David Gibson]

On Wed, Aug 28, 2019 at 07:20:53PM +0200, Greg Kurz wrote:
> A recent change in spapr_machine_reset() showed that resetting the compat
> mode in spapr_machine_reset() for the boot vCPU and in spapr_cpu_reset()
> for all other vCPUs was fragile. The fix was thus to reset the compat mode
> for all vCPUs in spapr_machine_reset(), but we still have to propagate
> it to hot-plugged CPUs. This is still performed from spapr_cpu_reset(),
> hence resulting in ppc_set_compat() being called twice for every vCPU at
> machine reset. Apart from wasting cycles, which isn't really an issue
> during machine reset, this seems to indicate that spapr_cpu_reset() isn't
> the best place to set the compat mode.
> 
> A natural candidate for CPU-hotplug specific code is spapr_core_plug().
> Also, it sits in the same file as spapr_machine_reset() : this makes
> it easier for someone who wants to know when the compat PVR is set.
> 
> Call ppc_set_compat() from there. This doesn't need to be done for
> initial vCPUs since the compat PVR is 0 and spapr_machine_reset() sets
> the appropriate value later. No need to do this on manually added vCPUS
> on the destination QEMU during migration since the compat PVR is
> part of the migrated vCPU state. Both conditions can be checked with
> spapr_drc_hotplugged().
> 
> Signed-off-by: Greg Kurz <address@hidden>

Applied, thanks.

> ---
>  hw/ppc/spapr.c          |   24 ++++++++++++++++--------
>  hw/ppc/spapr_cpu_core.c |    7 -------
>  2 files changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 447fb5c4eaaf..ea56499b4b3e 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -1758,12 +1758,6 @@ static void spapr_machine_reset(MachineState *machine)
>          spapr_ovec_cleanup(spapr->ov5_cas);
>          spapr->ov5_cas = spapr_ovec_new();
>  
> -        /*
> -         * reset compat_pvr for all CPUs
> -         * as qemu_devices_reset() is called before this,
> -         * it can't be propagated by spapr_cpu_reset()
> -         * from the first CPU to all the others
> -         */
>          ppc_set_compat_all(spapr->max_compat_pvr, &error_fatal);
>      }
>  
> @@ -3841,6 +3835,7 @@ static void spapr_core_plug(HotplugHandler 
> *hotplug_dev, DeviceState *dev,
>      CPUArchId *core_slot;
>      int index;
>      bool hotplugged = spapr_drc_hotplugged(dev);
> +    int i;
>  
>      core_slot = spapr_find_cpu_slot(MACHINE(hotplug_dev), cc->core_id, 
> &index);
>      if (!core_slot) {
> @@ -3874,13 +3869,26 @@ static void spapr_core_plug(HotplugHandler 
> *hotplug_dev, DeviceState *dev,
>      core_slot->cpu = OBJECT(dev);
>  
>      if (smc->pre_2_10_has_unused_icps) {
> -        int i;
> -
>          for (i = 0; i < cc->nr_threads; i++) {
>              cs = CPU(core->threads[i]);
>              pre_2_10_vmstate_unregister_dummy_icp(cs->cpu_index);
>          }
>      }
> +
> +    /*
> +     * Set compatibility mode to match the boot CPU, which was either set
> +     * by the machine reset code or by CAS.
> +     */
> +    if (hotplugged) {
> +        for (i = 0; i < cc->nr_threads; i++) {
> +            ppc_set_compat(core->threads[i], 
> POWERPC_CPU(first_cpu)->compat_pvr,
> +                           &local_err);
> +            if (local_err) {
> +                error_propagate(errp, local_err);
> +                return;
> +            }
> +        }
> +    }
>  }
>  
>  static void spapr_core_pre_plug(HotplugHandler *hotplug_dev, DeviceState 
> *dev,
> diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> index 45e2f2747ffc..1d93de8161f3 100644
> --- a/hw/ppc/spapr_cpu_core.c
> +++ b/hw/ppc/spapr_cpu_core.c
> @@ -41,13 +41,6 @@ static void spapr_cpu_reset(void *opaque)
>       * using an RTAS call */
>      cs->halted = 1;
>  
> -    /* Set compatibility mode to match the boot CPU, which was either set
> -     * by the machine reset code or by CAS. This should never fail.
> -     * At startup the value is already set for all the CPUs
> -     * but we need this when we hotplug a new CPU
> -     */
> -    ppc_set_compat(cpu, POWERPC_CPU(first_cpu)->compat_pvr, &error_abort);
> -
>      env->spr[SPR_HIOR] = 0;
>  
>      lpcr = env->spr[SPR_LPCR];
> 

-- 
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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