Linux Virtualization - Dec 2016 - [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

> Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for
> fast (de)inflating & fast live migration
> 
> On 12/14/2016 12:59 AM, Li, Liang Z wrote:
> >> Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend
virtio-balloon
> >> for fast (de)inflating & fast live migration
> >>
> >> On 12/08/2016 08:45 PM, Li, Liang Z wrote:
> >>> What's the conclusion of your discussion? It seems you
want some
> >>> statistic before deciding whether to  ripping the bitmap from
the
> >>> ABI, am I right?
> >>
> >> I think Andrea and David feel pretty strongly that we should
remove
> >> the bitmap, unless we have some data to support keeping it.  I
don't
> >> feel as strongly about it, but I think their critique of it is
pretty
> >> valid.  I think the consensus is that the bitmap needs to go.
> >>
> >> The only real question IMNHO is whether we should do a power-of-2
or
> >> a length.  But, if we have 12 bits, then the argument for doing
> >> length is pretty strong.  We don't need anywhere near 12 bits
if doing
> power-of-2.
> >
> > Just found the MAX_ORDER should be limited to 12 if use length instead
> > of order, If the MAX_ORDER is configured to a value bigger than 12, it
> > will make things more complex to handle this case.
> >
> > If use order, we need to break a large memory range whose length is
> > not the power of 2 into several small ranges, it also make the code
complex.
> 
> I can't imagine it makes the code that much more complex.  It adds a
for loop.
> Right?
> 
Yes, just a little. :)
> > 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?
Yes.

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.

> 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