Re: [Qemu-devel] balloon vs postcopy migrate

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

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> Hi,
>   Andrea pointed out there is a risk that a guest inflating its
> balloon during a postcopy migrate could cause us problems, and
> I wanted to see what the best way of avoiding the problem was.
> 
> Guests inflating there balloon cause an madvise(MADV_DONTNEED) on
> the host, marking pages as not present, that will potentially trigger
> a userfault, that we are using in postcopy to detect pages that need
> to be fetched from the source.
> 
> In theory, at the moment guests *should* only ask for a balloon
> inflation if they've been asked to do so by the host; however there
> are no guards for that, and it's been suggested giving the
> guest more freedom might be a good idea anyway.
> 
> My alternatives seem to be:
>    1) Stop servicing the message queue from the guest so
>      that we just don't notice the inflate messages until
>      afterwards.  (Easy for Qemu, not sure how the guests
>      will like an unserviced queue).
> 
>    2) I could keep servicing the queue and ignore the messages
>      (Easy for everyone, not very nice in actual used memory -
>       does it cause any long term problems other than that?)
> 
>    3) I could keep servicing the queue but put the messages
>      in a list somewhere that replay after migrate has finished.
>      (That list sounds bounded only in a very large way?)


As a follow up question; why is 'balloon_page' part of virtio-balloon.c
rather than balloon.c ?

I'm thinking of implementing (3) by putting a queue in front
of balloon_page, but it seems to make more sense to put that
type of thing in shared code (migration shouldn't need to know
it's virtio that's the transport?)

Dave

> 
> Thoughts?
> 
> Dave
> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK