> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
