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Subject: Re: [virtio] [OASIS Issue Tracker] Created: (VIRTIO-28) Implement new balloon device (ID 13)

Daniel Kiper <daniel.kiper@oracle.com> writes:
> On Thu, Dec 12, 2013 at 11:40:11AM +1030, Rusty Russell wrote:
>> If this is true, it means Xen cannot offer this device.  We can't have
>> Xen offer a device which a generic virtio-spec compliant OS can't drive.
>> We can, however, have a differently-numbered "Xen Balloon Device" which
>> takes xen-specific PFNs instead, and is otherwise identical.  This is
>> perfectly sane, and even the Linux driver (which will support both) will
>> look clean.
> This make sens but I think that it is last resort solution. IMO all
> devices/drivers should work on any hypervisor. I will play with new
> VIRTIO balloon device/driver and we will see what is possible.


>> It would have to be a feature bit: VIRTIO_BALLOON_F_UNLIMITED ?
> OK.
>> I think it would create an extra burden for non-Xen balloon devices:
>> - The PFN extracted from the balloon would either be a PFN which had
>>    been put into the balloon, or the lowest available PFN (ie. just
>>    above current top of mem)
>> Otherwise a guest might get a crazily sparse memory map.
>> Thoughts?
> I used this simplistic solution in Xen balloon driver and it does not
> work well if there is an I/O region above max_pfn. So we should not
> make this mistake here. I think that we could use solution similar
> to used on real hardware with ACPI. In memory hotplug case VIRTIO
> balloon device should send start address of hotplugged region and its
> size. Later everything should work as in pure balloon driver.

That makes perfect sense (though the name "UNLIMITED" now sucks... Hmm,

So if that feature is negotiated, then two new RO config fields become
available, which specify the range where the extra memory will appear?


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