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

[Li, Liang Z]

> On Thu, Dec 15, 2016 at 07:34:33AM -0800, Dave Hansen wrote:
> > 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?
> >
> > > 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?
> 
> I think you can probably assume page size >= 4K. But I would not want to
> make any other assumptions. E.g. there are systems that absolutely require
> you to set high bits for DMA.
> 
> I think we really want both length and order.
> 
> I understand how you are trying to pack them as tightly as possible.
> 
> However, I thought of a trick, we don't need to encode all possible orders.
> For example, with 2 bits of order, we can make them mean:
> 00 - 4K pages
> 01 - 2M pages
> 02 - 1G pages
> 
> guest can program the sizes for each order through config space.
> 
> We will have 10 bits left for legth.
> 

Please don't, we just get rid of the bitmap for simplification. :)

> It might make sense to also allow guest to program the number of bits used
> for order, this will make it easy to extend without host changes.
> 

There still exist the case if the MAX_ORDER is configured to a large value, 
e.g. 36 for a system
with huge amount of memory, then there is only 28 bits left for the pfn, which 
is not enough.
Should  we limit the MAX_ORDER? I don't think so.

It seems use order is better. 

Thanks!
Liang
> --
> MST