Re: [virtio] New virtio balloon...

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Mon, Feb 03, 2014 at 01:37:17PM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> > On Fri, Jan 31, 2014 at 04:01:39PM +1030, Rusty Russell wrote:
>> >> The driver needs to explicitly ask for pages in that region.
>> >
>> > OK so we'll have an extra flag for that?
>> 
>> No, I mean that the driver explicitly requests every page by PFN.  If it
>> requests a page in the extramem region, it will get one.
>
> Maybe I misunderstand. What is EXTRA_MEM is aid of?

The VIRTIO_BALLOON_F_EXTRA_MEM feature enables the extra memory region.
It's a simple form of memory hotplug.  Daniel wanted it.

It might make sense for some guests to allocate metadata (eg. array of
struct pages) for that extra memory if it's required.

> One other comment: for NUMA VMs we might want
> to have separate counters for each node,
> and separate memory pressure notification.
>
> Maybe just separate balloon for each node?

Good question.  We could add a range field in the config space, such
that the different balloons cover different areas.

On the other hand, we can add this in future with a feature bit.  Though
it would mean offering a guest multiple balloons, and then ignoring
all but one if it didn't understand the range feature.

Cheers,
Rusty.