Re: [Qemu-devel] [RFC] virtio-mem: paravirtualized memory

[Michael S. Tsirkin]

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