Re: [PATCH 1/4] migration/savevm: Add a compatibility check for capabilities

["Marco Cavenati" <Marco.Cavenati@eurecom.fr>]

Hello Fabiano,

First of all thanks a lot for the quick follow up to my issue!

I just want to point out that with only mapped-ram enabled (without
multifd) savevm/loadvm do not lead to a crash but just to an error
according to my (few) experiments (on upstream).

Ciao

Marco

On Thursday, March 27, 2025 15:39 CET, Fabiano Rosas <farosas@suse.de> wrote:

> It has always been possible to enable arbitrary migration capabilities
> and attempt to take a snapshot of the VM with the savevm/loadvm
> commands as well as their QMP counterparts
> snapshot-save/snapshot-load.
> 
> Most migration capabilities are not meant to be used with snapshots
> and there's a risk of crashing QEMU or producing incorrect
> behavior. Ideally, every migration capability would either be
> implemented for savevm or explicitly rejected.
> 
> Add a compatibility check routine and reject the snapshot command if
> an incompatible capability is enabled. For now only act on the the two
> that actually cause a crash: multifd and mapped-ram.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2881
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
>  migration/options.c | 26 ++++++++++++++++++++++++++
>  migration/options.h |  1 +
>  migration/savevm.c  |  8 ++++++++
>  3 files changed, 35 insertions(+)
> 
> diff --git a/migration/options.c b/migration/options.c
> index b0ac2ea408..8772b98dca 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -443,11 +443,37 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
>      MIGRATION_CAPABILITY_VALIDATE_UUID,
>      MIGRATION_CAPABILITY_ZERO_COPY_SEND);
>  
> +/* Snapshot compatibility check list */
> +static const
> +INITIALIZE_MIGRATE_CAPS_SET(check_caps_savevm,
> +                            MIGRATION_CAPABILITY_MULTIFD,
> +                            MIGRATION_CAPABILITY_MAPPED_RAM,
> +);
> +
>  static bool migrate_incoming_started(void)
>  {
>      return !!migration_incoming_get_current()->transport_data;
>  }
>  
> +bool migrate_can_snapshot(Error **errp)
> +{
> +    MigrationState *s = migrate_get_current();
> +    int i;
> +
> +    for (i = 0; i < check_caps_savevm.size; i++) {
> +        int incomp_cap = check_caps_savevm.caps[i];
> +
> +        if (s->capabilities[incomp_cap]) {
> +            error_setg(errp,
> +                       "Snapshots are not compatible with %s",
> +                       MigrationCapability_str(incomp_cap));
> +            return false;
> +        }
> +    }
> +
> +    return true;
> +}
> +
>  /**
>   * @migration_caps_check - check capability compatibility
>   *
> diff --git a/migration/options.h b/migration/options.h
> index 762be4e641..20b71b6f2a 100644
> --- a/migration/options.h
> +++ b/migration/options.h
> @@ -58,6 +58,7 @@ bool migrate_tls(void);
>  /* capabilities helpers */
>  
>  bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp);
> +bool migrate_can_snapshot(Error **errp);
>  
>  /* parameters */
>  
> diff --git a/migration/savevm.c b/migration/savevm.c
> index ce158c3512..3be13bcfe8 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -3229,6 +3229,10 @@ bool save_snapshot(const char *name, bool overwrite, const char *vmstate,
>  
>      GLOBAL_STATE_CODE();
>  
> +    if (!migrate_can_snapshot(errp)) {
> +        return false;
> +    }
> +
>      if (migration_is_blocked(errp)) {
>          return false;
>      }
> @@ -3413,6 +3417,10 @@ bool load_snapshot(const char *name, const char *vmstate,
>      int ret;
>      MigrationIncomingState *mis = migration_incoming_get_current();
>  
> +    if (!migrate_can_snapshot(errp)) {
> +        return false;
> +    }
> +
>      if (!bdrv_all_can_snapshot(has_devices, devices, errp)) {
>          return false;
>      }
> -- 
> 2.35.3
>