Re: [PATCH v17 6/7] migration: Include migration support for machine check handling

[David Gibson]

On Thu, Oct 24, 2019 at 01:13:06PM +0530, Ganesh Goudar wrote:
> From: Aravinda Prasad <address@hidden>
> 
> This patch includes migration support for machine check
> handling. Especially this patch blocks VM migration
> requests until the machine check error handling is
> complete as these errors are specific to the source
> hardware and is irrelevant on the target hardware.
> 
> [Do not set FWNMI cap in post_load, now its done in .apply hook]
> Signed-off-by: Ganesh Goudar <address@hidden>
> Signed-off-by: Aravinda Prasad <address@hidden>
> ---
>  hw/ppc/spapr.c         | 41 +++++++++++++++++++++++++++++++++++++++++
>  hw/ppc/spapr_events.c  | 16 +++++++++++++++-
>  hw/ppc/spapr_rtas.c    |  2 ++
>  include/hw/ppc/spapr.h |  2 ++
>  4 files changed, 60 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 346ec5ba6c..e0d0f95ec0 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -46,6 +46,7 @@
>  #include "migration/qemu-file-types.h"
>  #include "migration/global_state.h"
>  #include "migration/register.h"
> +#include "migration/blocker.h"
>  #include "mmu-hash64.h"
>  #include "mmu-book3s-v3.h"
>  #include "cpu-models.h"
> @@ -1751,6 +1752,8 @@ static void spapr_machine_reset(MachineState *machine)
>  
>      /* Signal all vCPUs waiting on this condition */
>      qemu_cond_broadcast(&spapr->mc_delivery_cond);
> +
> +    migrate_del_blocker(spapr->fwnmi_migration_blocker);
>  }
>  
>  static void spapr_create_nvram(SpaprMachineState *spapr)
> @@ -2041,6 +2044,43 @@ static const VMStateDescription vmstate_spapr_dtb = {
>      },
>  };
>  
> +static bool spapr_fwnmi_needed(void *opaque)
> +{
> +    SpaprMachineState *spapr = (SpaprMachineState *)opaque;
> +
> +    return spapr->guest_machine_check_addr != -1;
> +}
> +
> +static int spapr_fwnmi_pre_save(void *opaque)
> +{
> +    SpaprMachineState *spapr = (SpaprMachineState *)opaque;
> +
> +    /*
> +     * With -only-migratable QEMU option, we cannot block migration.
> +     * Hence check if machine check handling is in progress and print
> +     * a warning message.
> +     */

IIUC the logic below this could also occur in the case where the fwnmi
event occurs after a migration has started, but before it completes,
not just with -only-migratable.  Is that correct?

> +    if (spapr->mc_status != -1) {
> +        warn_report("A machine check is being handled during migration. The"
> +                "handler may run and log hardware error on the destination");
> +    }
> +
> +    return 0;
> +}
> +
> +static const VMStateDescription vmstate_spapr_machine_check = {
> +    .name = "spapr_machine_check",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .needed = spapr_fwnmi_needed,
> +    .pre_save = spapr_fwnmi_pre_save,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_UINT64(guest_machine_check_addr, SpaprMachineState),
> +        VMSTATE_INT32(mc_status, SpaprMachineState),
> +        VMSTATE_END_OF_LIST()
> +    },
> +};
> +
>  static const VMStateDescription vmstate_spapr = {
>      .name = "spapr",
>      .version_id = 3,
> @@ -2075,6 +2115,7 @@ static const VMStateDescription vmstate_spapr = {
>          &vmstate_spapr_cap_large_decr,
>          &vmstate_spapr_cap_ccf_assist,
>          &vmstate_spapr_cap_fwnmi,
> +        &vmstate_spapr_machine_check,
>          NULL
>      }
>  };
> diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c
> index db44e09154..30d9371c88 100644
> --- a/hw/ppc/spapr_events.c
> +++ b/hw/ppc/spapr_events.c
> @@ -43,6 +43,7 @@
>  #include "qemu/main-loop.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
> @@ -842,6 +843,8 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
>  {
>      SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
>      CPUState *cs = CPU(cpu);
> +    int ret;
> +    Error *local_err = NULL;
>  
>      if (spapr->guest_machine_check_addr == -1) {
>          /*
> @@ -871,8 +874,19 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
>              return;
>          }
>      }
> -    spapr->mc_status = cpu->vcpu_id;
>  
> +    ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, &local_err);
> +    if (ret == -EBUSY) {
> +        /*
> +         * We don't want to abort so we let the migration to continue.
> +         * In a rare case, the machine check handler will run on the target.
> +         * Though this is not preferable, it is better than aborting
> +         * the migration or killing the VM.
> +         */
> +        warn_report_err(local_err);

I suspect the error message in local_err won't be particularly
meaningful on its own.  Perhaps you need to add a prefix to clarify
that the problem is you've received a fwnmi after migration has
commenced?

> +    }
> +
> +    spapr->mc_status = cpu->vcpu_id;
>      spapr_mce_dispatch_elog(cpu, recovered);
>  }
>  
> diff --git a/hw/ppc/spapr_rtas.c b/hw/ppc/spapr_rtas.c
> index 0328b1f341..c78d96ee7e 100644
> --- a/hw/ppc/spapr_rtas.c
> +++ b/hw/ppc/spapr_rtas.c
> @@ -50,6 +50,7 @@
>  #include "hw/ppc/fdt.h"
>  #include "target/ppc/mmu-hash64.h"
>  #include "target/ppc/mmu-book3s-v3.h"
> +#include "migration/blocker.h"
>  
>  static void rtas_display_character(PowerPCCPU *cpu, SpaprMachineState *spapr,
>                                     uint32_t token, uint32_t nargs,
> @@ -446,6 +447,7 @@ static void rtas_ibm_nmi_interlock(PowerPCCPU *cpu,
>       */
>      spapr->mc_status = -1;
>      qemu_cond_signal(&spapr->mc_delivery_cond);
> +    migrate_del_blocker(spapr->fwnmi_migration_blocker);

Oh... damn.  I suggested using a static fwnmi_migration_blocker
instance, but I just realized there's a problem with it.

If we do receive multiple fwnmi events on different cpus at roughly
the same time, this will break: I think we'll try to add the same
migration blocker instance multiple times, which won't be good.  Even
if that doesn't do anything *too* bad, then we'll unblock the
migration on the first interlock, rather than waiting for all pending
fwnmi events to complete.

>      rtas_st(rets, 0, RTAS_OUT_SUCCESS);
>  }
>  
> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> index 86f0fc8fdd..290abd6328 100644
> --- a/include/hw/ppc/spapr.h
> +++ b/include/hw/ppc/spapr.h
> @@ -215,6 +215,8 @@ struct SpaprMachineState {
>  
>      unsigned gpu_numa_id;
>      SpaprTpmProxy *tpm_proxy;
> +
> +    Error *fwnmi_migration_blocker;
>  };
>  
>  #define H_SUCCESS         0

-- 
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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