Re: [PATCH 4/5] RFC: migration: check required subsections are loaded, once

[Peter Xu <peterx@redhat.com>]

On Tue, Sep 26, 2023 at 07:59:24PM +0400, marcandre.lureau@redhat.com wrote:
> @@ -484,6 +513,13 @@ static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription *vmsd,
>          }
>      }
>  
> +    for (i = 0; i < n; i++) {
> +        if (!visited[i] && vmstate_save_needed(vmsd->subsections[i], opaque)) {
> +            trace_vmstate_subsection_load_bad(vmsd->name, vmsd->subsections[i]->name, "(not visited)");
> +            return -ENOENT;
> +        }
> +    }

One thing that might be tricky to call needed() on loading side is, IMHO
the needed() hooks normally was designed to be only called on a complete VM
state. IOW, I think it can reference any machine/device state, or whatever
variable assuming all of them contain valid data.

But the load side may not yet contain everything..  we can guarantee here
we loaded the full device state of this one as subsections should be the
last to come, and all we've loaded so far.  But what if it references
something else outside what we've loaded?  It looks possible in some
special .needed() hook we can return something unexpected.

I assume most needed() hooks are fine (and it does look like we can find
bugs with this, which means this might be proved useful already at least in
some form or another). I just worry on something start to break after we
become strict on this.

Maybe.. make the check only throw warnings, but not yet fail the migration?

> +
>      trace_vmstate_subsection_load_good(vmsd->name);
>      return 0;
>  }
> -- 
> 2.41.0
> 

-- 
Peter Xu