Re: [PATCH v2 03/16] migration: make .post_save() a void function

[Fabiano Rosas <farosas@suse.de>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> All other handlers now have _errp() variants. Should we go this way
> for .post_save()? Actually it's rather strange, when the vmstate do
> successful preparations in .pre_save(), then successfully save all
> sections and subsections, end then fail when all the state is
> successfully transferred to the target.
>
> Happily, we have only three .post_save() realizations, all always
> successful. Let's make this a rule.
>
> Also note, that we call .post_save() in two places, and handle
> its (theoretical) failure inconsistently. Fix that too.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> ---
>  docs/devel/migration/main.rst |  2 +-
>  hw/ppc/spapr_pci.c            |  3 +--
>  include/migration/vmstate.h   | 10 +++++++++-
>  migration/savevm.c            |  3 +--
>  migration/vmstate.c           | 12 +++---------
>  target/arm/machine.c          |  4 +---
>  6 files changed, 16 insertions(+), 18 deletions(-)
>
> diff --git a/docs/devel/migration/main.rst b/docs/devel/migration/main.rst
> index 234d280249..2de7050764 100644
> --- a/docs/devel/migration/main.rst
> +++ b/docs/devel/migration/main.rst
> @@ -439,7 +439,7 @@ The functions to do that are inside a vmstate definition, and are called:
>  
>    This function is called before we save the state of one device.
>  
> -- ``int (*post_save)(void *opaque);``
> +- ``void (*post_save)(void *opaque);``
>  
>    This function is called after we save the state of one device
>    (even upon failure, unless the call to pre_save returned an error).
> diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> index ea998bdff1..1dc3b02659 100644
> --- a/hw/ppc/spapr_pci.c
> +++ b/hw/ppc/spapr_pci.c
> @@ -2093,14 +2093,13 @@ static int spapr_pci_pre_save(void *opaque)
>      return 0;
>  }
>  
> -static int spapr_pci_post_save(void *opaque)
> +static void spapr_pci_post_save(void *opaque)
>  {
>      SpaprPhbState *sphb = opaque;
>  
>      g_free(sphb->msi_devs);
>      sphb->msi_devs = NULL;
>      sphb->msi_devs_num = 0;
> -    return 0;
>  }
>  
>  static int spapr_pci_post_load(void *opaque, int version_id)
> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> index 3695afd483..15578b3e28 100644
> --- a/include/migration/vmstate.h
> +++ b/include/migration/vmstate.h
> @@ -223,7 +223,15 @@ struct VMStateDescription {
>      bool (*post_load_errp)(void *opaque, int version_id, Error **errp);
>      int (*pre_save)(void *opaque);
>      bool (*pre_save_errp)(void *opaque, Error **errp);
> -    int (*post_save)(void *opaque);
> +
> +    /*
> +     * post_save() rarely used to free some temporary resources.
> +     * It's is called if .pre_save[_errp]() call was successful
> +     * (or .pre_save[_errp] handler absent), regardless success
> +     * or failure during fields and subsections save. If
> +     * .pre_save[_errp]() fails, .post_save() is not called.
> +     */

I would not mention usage directly and maybe also simplify the language
a bit. If there's doubt on the exact flow, people can read the code:

  /*
   * Unless .pre_save() fails, post_save() is called after saving
   * fields and subsections. It should not fail because at this point
   * the state has potentially already been transferred.
   */

> +    void (*post_save)(void *opaque);
>      bool (*needed)(void *opaque);
>      bool (*dev_unplug_pending)(void *opaque);
>  
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 3a16c467b2..c5236e71ba 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -321,14 +321,13 @@ static int configuration_pre_save(void *opaque)
>      return 0;
>  }
>  
> -static int configuration_post_save(void *opaque)
> +static void configuration_post_save(void *opaque)
>  {
>      SaveState *state = opaque;
>  
>      g_free(state->capabilities);
>      state->capabilities = NULL;
>      state->caps_count = 0;
> -    return 0;
>  }
>  
>  static int configuration_pre_load(void *opaque)
> diff --git a/migration/vmstate.c b/migration/vmstate.c
> index 651c3fe011..5111e7a141 100644
> --- a/migration/vmstate.c
> +++ b/migration/vmstate.c
> @@ -550,10 +550,7 @@ static int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>                  if (ret) {
>                      error_prepend(errp, "Save of field %s/%s failed: ",
>                                    vmsd->name, field->name);
> -                    if (vmsd->post_save) {
> -                        vmsd->post_save(opaque);
> -                    }
> -                    return ret;
> +                    goto out;
>                  }
>  
>                  /* Compressed arrays only care about the first element */
> @@ -578,12 +575,9 @@ static int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>  
>      ret = vmstate_subsection_save(f, vmsd, opaque, vmdesc, errp);
>  
> +out:
>      if (vmsd->post_save) {
> -        int ps_ret = vmsd->post_save(opaque);
> -        if (!ret && ps_ret) {
> -            ret = ps_ret;
> -            error_setg(errp, "post-save failed: %s", vmsd->name);
> -        }
> +        vmsd->post_save(opaque);

Nice cleanup.

>      }
>      return ret;
>  }
> diff --git a/target/arm/machine.c b/target/arm/machine.c
> index bbaae34449..de810220e2 100644
> --- a/target/arm/machine.c
> +++ b/target/arm/machine.c
> @@ -993,15 +993,13 @@ static int cpu_pre_save(void *opaque)
>      return 0;
>  }
>  
> -static int cpu_post_save(void *opaque)
> +static void cpu_post_save(void *opaque)
>  {
>      ARMCPU *cpu = opaque;
>  
>      if (!kvm_enabled()) {
>          pmu_op_finish(&cpu->env);
>      }
> -
> -    return 0;
>  }
>  
>  static int cpu_pre_load(void *opaque)