RE: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

From: Li, Liang Z
Date: Thu Dec 15 2016 - 20:38:10 EST

> On 12/15/2016 04:48 PM, Li, Liang Z wrote:
> >>> It seems we leave too many bit for the pfn, and the bits leave for
> >>> length is not enough, How about keep 45 bits for the pfn and 19 bits
> >>> for length, 45 bits for pfn can cover 57 bits physical address, that
> >>> should be
> >> enough in the near feature.
> >>> What's your opinion?
> >> I still think 'order' makes a lot of sense. But, as you say, 57 bits
> >> is enough for
> >> x86 for a while. Other architectures.... who knows?
> Thinking about this some more... There are really only two cases that
> matter: 4k pages and "much bigger" ones.
> Squeezing each 4k page into 8 bytes of metadata helps guarantee that this
> scheme won't regress over the old scheme in any cases. For bigger ranges, 8
> vs 16 bytes means *nothing*. And 16 bytes will be as good or better than
> the old scheme for everything which is >4k.
> How about this:
> * 52 bits of 'pfn', 5 bits of 'order', 7 bits of 'length'
> * One special 'length' value to mean "actual length in next 8 bytes"
> That should be pretty simple to produce and decode. We have two record
> sizes, but I think it is manageable.

It works, Now that we intend to use another 8 bytes for length

Why not:

Use 52 bits for 'pfn', 12 bits for 'length', when the 12 bits is not long enough for the 'length'
Set the 'length' to a special value to indicate the "actual length in next 8 bytes".

That will be much more simple. Right?