Re: [Qemu-devel] [PATCH v5 6/6] migration: Block migration while handling machine check

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

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On Thu, Sep 28, 2017 at 04:08:31PM +0530, Aravinda Prasad wrote:
> Block VM migration requests until the machine check
> error handling is complete as (i) these errors are
> specific to the source hardware and is irrelevant on
> the target hardware, (ii) these errors cause data
> corruption and should be handled before migration.
> 
> Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
> ---
>  hw/ppc/spapr_rtas.c    |    3 +++
>  include/hw/ppc/spapr.h |    2 ++
>  target/ppc/kvm.c       |   17 +++++++++++++++++
>  3 files changed, 22 insertions(+)
> 
> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
> index d017a67..17f6567 100644
> --- a/hw/ppc/spapr_rtas.c
> +++ b/hw/ppc/spapr_rtas.c
> @@ -47,6 +47,7 @@
>  #include "trace.h"
>  #include "hw/ppc/fdt.h"
>  #include "kvm_ppc.h"
> +#include "migration/blocker.h"
>  
>  static void rtas_display_character(PowerPCCPU *cpu, sPAPRMachineState *spapr,
>                                     uint32_t token, uint32_t nargs,
> @@ -390,6 +391,8 @@ static void rtas_ibm_nmi_interlock(PowerPCCPU *cpu,
>          spapr->mc_status = -1;
>          qemu_cond_signal(&spapr->mc_delivery_cond);
>          rtas_st(rets, 0, RTAS_OUT_SUCCESS);
> +        migrate_del_blocker(spapr->migration_blocker);
> +        error_free(spapr->migration_blocker);
>      }
>  }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index a75e9cf..0890a44 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -7,6 +7,7 @@
>  #include "hw/ppc/spapr_drc.h"
>  #include "hw/mem/pc-dimm.h"
>  #include "hw/ppc/spapr_ovec.h"
> +#include "qapi/error.h"
>  
>  struct VIOsPAPRBus;
>  struct sPAPRPHBState;
> @@ -136,6 +137,7 @@ struct sPAPRMachineState {
>      MemoryHotplugState hotplug_memory;
>  
>      const char *icp_type;
> +    Error *migration_blocker;

This isn't a good name, because it's _specifically_ the fwnmi as a
migration blocker - trying to put another migration blocker in here
would break horribly, because nmi-interlock would clear it regardless.

>  };
>  
>  #define H_SUCCESS         0
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 59b3322..58de7ea 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -52,6 +52,7 @@
>  #endif
>  #include "elf.h"
>  #include "sysemu/kvm_int.h"
> +#include "migration/blocker.h"
>  
>  //#define DEBUG_KVM
>  
> @@ -2770,10 +2771,26 @@ int kvm_handle_nmi(PowerPCCPU *cpu, struct kvm_run *run)
>      sPAPRMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
>      PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
>      target_ulong msr = 0;
> +    Error *local_err = NULL;
> +    int ret;
>      bool type, le;
>  
>      cpu_synchronize_state(CPU(cpu));
>  
> +    error_setg(&spapr->migration_blocker,
> +            "Live migration not supported during machine check error handling");

In fact, there's no real reason to generate the error here.  The
error's always the same so you could just create it at startup as a
global and just add/remove it to the block list.

> +    ret = migrate_add_blocker(spapr->migration_blocker, &local_err);
> +    if (ret < 0) {
> +        /*
> +         * We don't want to abort and let the migration to continue. In a
> +         * rare case, the machine check handler will run on the target
> +         * hardware. Though this is not preferable, it is better than aborting

Why is it not preferable?  I mean it's an edge case, but AFAICT it's
still the correct behaviour.

> +         * the migration or killing the VM.
> +         */
> +        error_free(spapr->migration_blocker);
> +        fprintf(stderr, "Warning: Machine check during VM migration\n");

Use error_report(), not fprintf().

> +    }
> +
>      /*
>       * Properly set bits in MSR before we invoke the handler.
>       * SRR0/1, DAR and DSISR are properly set by KVM
> 

-- 
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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