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Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER
Date: Fri, 10 Mar 2017 21:35:21 +0200

On Fri, Mar 10, 2017 at 09:11:44AM -0800, Matthew Wilcox wrote:
> On Fri, Mar 10, 2017 at 05:58:28PM +0200, Michael S. Tsirkin wrote:
> > One of the issues of current balloon is the 4k page size
> > assumption. For example if you free a huge page you
> > have to split it up and pass 4k chunks to host.
> > Quite often host can't free these 4k chunks at all (e.g.
> > when it's using huge tlb fs).
> > It's even sillier for architectures with base page size >4k.
> I completely agree with you that we should be able to pass a hugepage
> as a single chunk.  Also we shouldn't assume that host and guest have
> the same page size.  I think we can come up with a scheme that actually
> lets us encode that into a 64-bit word, something like this:
> 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?

> (note this is a single page of order N, so we leave the page count bits
> set to 0, meaning one page).
> > Two things to consider:
> > - 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.

> > - guest should pass full huge pages to host
> >   this should be done correctly to avoid breaking up huge pages
> >   I would say yes let's use a single format but drop the "normal chunk"
> >   and always use the extended one.
> >   Also, size is in units of 4k, right? Please document that low 12 bit
> >   are reserved, they will be handy as e.g. flags.
> 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.


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