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Subject: Re: [virtio-comment] [PATCH 1/3] shared memory: Define shared memory regions

On Fri, 15 Feb 2019 12:26:00 +0100
David Hildenbrand <david@redhat.com> wrote:

> Probing is always ugly. But I think we can add something like
>  the x86 PCI hole between 3 and 4 GB after our initial boot memory.
> So there, we would have a memory region just like e.g. x86 has.

A special region is probably the best way out of this pickle. We would
only need the discovery ccw for virtio, then.

> This should even work with other mechanism I am working on. E.g.
> for memory devices, we will add yet another memory region above
> the special PCI region.
> The layout of the guest would then be something like
> [0x000000000000000]
> ... Memory region containing RAM
> [ram_size         ]
> ... Memory region for e.g. special PCI devices
> [ram_size +1 GB   ]
> ... Memory region for memory devices (virtio-pmem, virtio-mem ...)
> [maxram_size - ram_size + 1GB]
> We would have to create proper page tables for guest backing that take
> care of the new guest size (not just ram_size). Also, to the guest we
> would indicate "maximum ram size == ram_size" so it does not try to
> probe the "special" memory.

Hm... so that would be:
- 0..ram_size: just like it is handled now
- ram_size..ram_size + 1GB: guest does not treat it as ram, but does
  build page tables for it
- ram_size + 1GB..maxram_size: for whatever memory devices do with it

How does the guest probe this? (SCLP?) Or does the guest simply know
via some kind of probable feature that there's a 1GB region there?

> As we are using paravirtualized features here, this should be fine for.
> Unmodified guests will never touch/probe anything beyond ram_size.

Yes, that gives us more freedom.

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