On Fri, Jun 16, 2017 at 04:20:02PM +0200, David Hildenbrand wrote:

Hi,

this is an idea that is based on Andrea Arcangeli's original idea to
host enforce guest access to memory given up using virtio-balloon using
userfaultfd in the hypervisor. While looking into the details, I
realized that host-enforcing virtio-balloon would result in way too many
problems (mainly backwards compatibility) and would also have some
conceptual restrictions that I want to avoid. So I developed the idea of
virtio-mem - "paravirtualized memory".

Thanks! I went over this quickly, will read some more in the
coming days. I would like to ask for some clarifications
on one part meanwhile:

Q: Why not reuse virtio-balloon?

A: virtio-balloon is for cooperative memory management. It has a fixed
   page size

We are fixing that with VIRTIO_BALLOON_F_PAGE_CHUNKS btw.
I would appreciate you looking into that patchset.

and will deflate in certain situations.

What does this refer to?

Any change we
   introduce will break backwards compatibility.

Why does this have to be the case?

virtio-balloon was not
   designed to give guarantees. Nobody can hinder the guest from
   deflating/reusing inflated memory.

Reusing without deflate is forbidden with TELL_HOST, right?

   In addition, it might make perfect
   sense to have both, virtio-balloon and virtio-mem at the same time,
   especially looking at the DEFLATE_ON_OOM or STATS features of
   virtio-balloon. While virtio-mem is all about guarantees, virtio-
   balloon is about cooperation.

Thanks, and I intend to look more into this next week.

-- 
MST

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