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

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Date:   Fri, 16 Dec 2016 01:38:01 +0000
From:   "Li, Liang Z" <liang.z.li@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>
CC:     David Hildenbrand <david@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dgilbert@...hat.com" <dgilbert@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: RE: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for
 fast (de)inflating & fast live migration

> 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?

Liang