On Fri, Mar 10, 2017 at 01:25:41PM -0800, Matthew Wilcox wrote:
> On Fri, Mar 10, 2017 at 09:35:21PM +0200, Michael S. Tsirkin wrote:
> > > bit 0 clear => bits 1-11 encode a page count, bits 12-63 encode a PFN,
> > > page size 4k.
> > > bit 0 set, bit 1 clear => bits 2-12 encode a page count, bits 13-63
> > > encode a PFN, page size 8k
> > > bits 0+1 set, bit 2 clear => bits 3-13 for page count, bits 14-63 for
> > > PFN, page size 16k.
> > > bits 0-2 set, bit 3 clear => bits 4-14 for page count, bits 15-63 for
> > > PFN, page size 32k
> > > bits 0-3 set, bit 4 clear => bits 5-15 for page count, bits 16-63 for
> > > PFN, page size 64k
> > > That means we can always pass 2048 pages (of whatever page size) in a
> > > single chunk. And
> > > we support arbitrary power of two page sizes. I suggest something like
> > > this:
> > >
> > > u64 page_to_chunk(struct page *page)
> > > {
> > > u64 chunk = page_to_pfn(page) << PAGE_SHIFT;
> > > chunk |= (1UL << compound_order(page)) - 1;
> > > }
> >
> > You need to fill in the size, do you not?
>
> I think I did ... (1UL << compound_order(page)) - 1 sets the bottom
> N bits. Bit N+1 will already be clear. What am I missing?
This sets the order but not the number of pages.
For that you would do something like
chunk |= size << compound_order(page)
right?
> > > > - host should pass its base page size to guest
> > > > this can be a separate patch and for now we can fall back on 12 bit
> > > > if not there
> > >
> > > With this encoding scheme, I don't think we need to do this? As long as
> > > it's *at least* 12 bit, then we're fine.
> >
> > I think we will still need something like this down the road. The point
> > is that not all hosts are able to use 4k pages in a balloon.
> > So it's pointless for guest to pass 4k pages to such a host,
> > and we need host to tell guest the page size it needs.
> >
> > However that's a separate feature that can wait until
> > another day.
>
> Ah, the TRIM/DISCARD debate all over again ... should the guest batch
> up or should the host do that work ... probably easier to account it in
> the guest. Might be better to frame it as 'balloon chunk size' rather than
> host page size as it might have nothing to do with the host page size.
Exactly.
> > > What per-chunk flags are you thinking would be useful?
> >
> > Not entirely sure but I think would have been prudent to leave some free
> > if possible. Your encoding seems to use them all up, so be it.
>
> We don't necessarily have to support 2048 pages in a single chunk.
> If it's worth reserving some bits, we can do that at the expense of
> reducing the maximum number of pages per chunk.
Well we can always change things with a feature bit ..
I'll leave this up to you and Wei.
--
MST
