Re: [Qemu-devel] [RFC 28/29] migration: final handshake for the resume

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> Finish the last step to do the final handshake for the recovery.
> 
> First source sends one MIG_CMD_RESUME to dst, telling that source is
> ready to resume.
> 
> Then, dest replies with MIG_RP_MSG_RESUME_ACK to source, telling that
> dest is ready to resume (after switch to postcopy-active state).
> 
> When source received the RESUME_ACK, it switches its state to
> postcopy-active, and finally the recovery is completed.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c | 39 +++++++++++++++++++++++++++++++++++----
>  1 file changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 93fbc96..ecebe30 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1666,6 +1666,13 @@ static int migrate_handle_rp_recv_bitmap(MigrationState *s, char *block_name)
>      return ram_dirty_bitmap_reload(s, block);
>  }
>  
> +static void postcopy_resume_handshake_ack(MigrationState *s)
> +{
> +    qemu_mutex_lock(&s->resume_lock);
> +    qemu_cond_signal(&s->resume_cond);
> +    qemu_mutex_unlock(&s->resume_lock);
> +}
> +
>  static int migrate_handle_rp_resume_ack(MigrationState *s, uint32_t value)
>  {
>      trace_source_return_path_thread_resume_ack(value);
> @@ -1680,7 +1687,8 @@ static int migrate_handle_rp_resume_ack(MigrationState *s, uint32_t value)
>      migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_RECOVER,
>                        MIGRATION_STATUS_POSTCOPY_ACTIVE);
>  
> -    /* TODO: notify send thread that time to continue send pages */
> +    /* Notify send thread that time to continue send pages */
> +    postcopy_resume_handshake_ack(s);
>  
>      return 0;
>  }
> @@ -2151,6 +2159,25 @@ static bool postcopy_should_start(MigrationState *s)
>      return atomic_read(&s->start_postcopy) || s->start_postcopy_fast;
>  }
>  
> +static int postcopy_resume_handshake(MigrationState *s)
> +{
> +    qemu_mutex_lock(&s->resume_lock);
> +
> +    qemu_savevm_send_postcopy_resume(s->to_dst_file);
> +
> +    while (s->state == MIGRATION_STATUS_POSTCOPY_RECOVER) {
> +        qemu_cond_wait(&s->resume_cond, &s->resume_lock);
> +    }
> +
> +    qemu_mutex_unlock(&s->resume_lock);
> +
> +    if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> +        return 0;
> +    }

That feels to be a small racy - couldn't that validly become a
MIGRATION_STATUS_COMPLETED before that check?

I wonder if we need to change migrate_fd_cancel to be able to
cause a cancel in this case?

Dave

> +    return -1;
> +}
> +
>  /* Return zero if success, or <0 for error */
>  static int postcopy_do_resume(MigrationState *s)
>  {
> @@ -2168,10 +2195,14 @@ static int postcopy_do_resume(MigrationState *s)
>      }
>  
>      /*
> -     * TODO: handshake with dest using MIG_CMD_RESUME,
> -     * MIG_RP_MSG_RESUME_ACK, then switch source state to
> -     * "postcopy-active"
> +     * Last handshake with destination on the resume (destination will
> +     * switch to postcopy-active afterwards)
>       */
> +    ret = postcopy_resume_handshake(s);
> +    if (ret) {
> +        error_report("%s: handshake failed: %d", __func__, ret);
> +        return ret;
> +    }
>  
>      return 0;
>  }
> -- 
> 2.7.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK