linux-kernel - Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK

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Date:   Sun, 12 Mar 2017 02:05:47 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wei Wang <wei.w.wang@...el.com>, virtio-dev@...ts.oasis-open.org,
        kvm@...r.kernel.org, qemu-devel@...gnu.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Liang Li <liang.z.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        Amit Shah <amit.shah@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Liang Li <liliang324@...il.com>
Subject: Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of
 VIRTIO_BALLOON_F_CHUNK_TRANSFER

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