comment to guest statistics available through virtio balloon (5.5.6.3 and 5.5.6.4)

["Denis V. Lunev" <den@openvz.org>]

Hello, All!

During work with the VirtIO balloon we have found that it would be quite
useful to add additional field to balloon statistics, namely 
VIRTIO_BALLOON_S_AVAIL,
corresponding to 'Available' in /proc/meminfo.

It indicates to the hypervisor how big the balloon can be inflated without
pushing the guest system to swap. This metric would be very useful in VM
orchestration software to improve memory management of different VMs
under overcommit.

Thus the following changes are proposed to the standard into sections
5.5.6.3 and 5.5.6.4.

Section 5.5.6.3 should look have additional line:
structvirtio_balloon_stat{
#defineVIRTIO_BALLOON_S_SWAP_IN0
#defineVIRTIO_BALLOON_S_SWAP_OUT1
#defineVIRTIO_BALLOON_S_MAJFLT2
#defineVIRTIO_BALLOON_S_MINFLT3
#defineVIRTIO_BALLOON_S_MEMFREE4
#defineVIRTIO_BALLOON_S_MEMTOT5
#defineVIRTIO_BALLOON_S_AVAIL 6     <----
le16tag;
le64val;
}__attribute__((packed));

Section 5.5.6.4 should have an additional clause:

VIRTIO_BALLOON_S_AVAIL(6)
   The amount of memory free for new workload in the guest without
   pushing the system into swap.

Thank you in advance,
    Den