Re: [Qemu-devel] [PATCH] migration: Introduce migration_in_completion()

[Juan Quintela]

Pavel Fedin <address@hidden> wrote:
>  Hello!
>
>> Power people have a similar problem with its hashed page tables, they
>> integrated their own save_live implementation because they are too big
>> for the last stage.  You can look there for inspiration.
>
>  I examined their code. Interesting, and, indeed, it opens up a way
> for decreasing downtime by implementing iterative migration for
> the ITS.
>  However, this is not really what is necessary. This thing aims to
> produce own data chunk, and it's not good for ITS. ITS already
> stores everything in system RAM, therefore savevm_ram_handlers take
> perfect care about these data. The only thing to do is to tell
> the ITS to dump its state into RAM. This is what i currently do using
> migration_in_completion().
>  An alternate, perhaps better approach, would be to be able to hook
> into ram_save_iterate() and ram_save_complete(). This way we
> could kick ITS right before attempting to migrate RAM.
>  Could we extend the infrastructure so that:
> a) Handlers are prioritized, and we can determine order of their execution?
> b) We can choose whether our handlers actually produce extra chunk or not?
>
>  OTOH, what i've done is actually a way to hook up into
> save_live_complete before any other registered handlers get
> executed. What
> is missing is one more notifier_list_notify() call right before
> qemu_savevm_state_iterate(), and a corresponding
> migration_is_active() checker.
>
>  What do you think ?

ok, your problem here is that you modify ram.  Could you take a look at
how vhost manage this?  It is done at migration_bitmap_sync(), and it
just marks the pages that are dirty.

The "just" part in the interesting bit.  It uses the memory_region
operations.  Michael, do you understand that code better than me, could
you give an introduction and say if it would work for the ITS?  Thanks.


Later, Juan.