Re: [Qemu-devel] [Qemu-ppc] [PATCH v8 6/6] migration: Block migration while handling machine check

[Greg Kurz <groug@kaod.org>]

On Mon, 22 Apr 2019 12:33:45 +0530
Aravinda Prasad <aravinda@linux.vnet.ibm.com> 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_events.c  |   17 +++++++++++++++++
>  hw/ppc/spapr_rtas.c    |    4 ++++
>  include/hw/ppc/spapr.h |    3 +++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c
> index 4032db0..45b990c 100644
> --- a/hw/ppc/spapr_events.c
> +++ b/hw/ppc/spapr_events.c
> @@ -41,6 +41,7 @@
>  #include "qemu/bcd.h"
>  #include "hw/ppc/spapr_ovec.h"
>  #include <libfdt.h>
> +#include "migration/blocker.h"
>  
>  #define RTAS_LOG_VERSION_MASK                   0xff000000
>  #define   RTAS_LOG_VERSION_6                    0x06000000
> @@ -864,6 +865,22 @@ static void spapr_mce_dispatch_elog(PowerPCCPU *cpu, bool recovered)
>  void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
>  {
>      SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
> +    int ret;
> +    Error *local_err = NULL;
> +
> +    error_setg(&spapr->migration_blocker,
> +            "Live migration not supported during machine check handling");
> +    ret = migrate_add_blocker(spapr->migration_blocker, &local_err);

migrate_add_blocker() propagates the reason of the failure in local_err,
ie. because a migration is already in progress or --only-migratable was
passed on the QEMU command line, along with the error message passed in
the first argument. This means that...

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

... you should just do:

        error_report_err(local_err);

This also takes care of freeing local_err which would be leaked otherwise.

> +    }
>  
>      while (spapr->mc_status != -1) {
>          /*
> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
> index 997cf19..1229a0e 100644
> --- a/hw/ppc/spapr_rtas.c
> +++ b/hw/ppc/spapr_rtas.c
> @@ -50,6 +50,7 @@
>  #include "target/ppc/mmu-hash64.h"
>  #include "target/ppc/mmu-book3s-v3.h"
>  #include "kvm_ppc.h"
> +#include "migration/blocker.h"
>  
>  static void rtas_display_character(PowerPCCPU *cpu, SpaprMachineState *spapr,
>                                     uint32_t token, uint32_t nargs,
> @@ -396,6 +397,9 @@ 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);
> +        spapr->migration_blocker = NULL;
>      }
>  }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index 9d16ad1..dda5fd2 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -10,6 +10,7 @@
>  #include "hw/ppc/spapr_irq.h"
>  #include "hw/ppc/spapr_xive.h"  /* For SpaprXive */
>  #include "hw/ppc/xics.h"        /* For ICSState */
> +#include "qapi/error.h"
>  
>  struct SpaprVioBus;
>  struct SpaprPhbState;
> @@ -213,6 +214,8 @@ struct SpaprMachineState {
>      SpaprCapabilities def, eff, mig;
>  
>      unsigned gpu_numa_id;
> +
> +    Error *migration_blocker;
>  };
>  
>  #define H_SUCCESS         0
> 
>