From: "Michael S. Tsirkin" <mst@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Wei Wang <wei.w.wang@intel.com>,
virtio-dev@lists.oasis-open.org, kvm@vger.kernel.org,
qemu-devel@nongnu.org, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org, linux-mm@kvack.org,
Liang Li <liang.z.li@intel.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Cornelia Huck <cornelia.huck@de.ibm.com>,
Amit Shah <amit.shah@redhat.com>,
Dave Hansen <dave.hansen@intel.com>,
Andrea Arcangeli <aarcange@redhat.com>,
David Hildenbrand <david@redhat.com>,
Liang Li <liliang324@gmail.com>
Subject: Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER
Date: Sun, 12 Mar 2017 02:05:47 +0200 [thread overview]
Message-ID: <20170312020203-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20170310212541.GC16328@bombadil.infradead.org>
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
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next prev parent reply other threads:[~2017-03-12 0:05 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-03 5:40 [PATCH v7 kernel 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration Wei Wang
2017-03-03 5:40 ` [PATCH v7 kernel 1/5] virtio-balloon: rework deflate to add page to a list Wei Wang
2017-03-03 5:40 ` [PATCH v7 kernel 2/5] virtio-balloon: VIRTIO_BALLOON_F_CHUNK_TRANSFER Wei Wang
2017-03-08 4:01 ` Michael S. Tsirkin
2017-03-09 7:12 ` Wei Wang
2017-03-03 5:40 ` [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER Wei Wang
2017-03-08 4:01 ` Michael S. Tsirkin
2017-03-10 10:02 ` [virtio-dev] " Wei Wang
2017-03-10 13:26 ` David Hildenbrand
2017-03-10 15:37 ` Michael S. Tsirkin
2017-03-09 14:14 ` Matthew Wilcox
2017-03-10 11:37 ` Wei Wang
2017-03-10 15:58 ` Michael S. Tsirkin
2017-03-10 17:11 ` Matthew Wilcox
2017-03-10 19:10 ` Michael S. Tsirkin
2017-03-10 21:18 ` Matthew Wilcox
2017-03-10 19:35 ` Michael S. Tsirkin
2017-03-10 21:25 ` Matthew Wilcox
2017-03-12 0:05 ` Michael S. Tsirkin [this message]
2017-03-11 11:59 ` Wei Wang
2017-03-11 14:09 ` Matthew Wilcox
2017-03-12 1:59 ` Wang, Wei W
2017-03-12 4:04 ` Michael S. Tsirkin
2017-03-13 12:41 ` Wang, Wei W
2017-03-12 0:07 ` Michael S. Tsirkin
2017-03-03 5:40 ` [PATCH v7 kernel 4/5] virtio-balloon: define flags and head for host request vq Wei Wang
2017-03-08 4:02 ` Michael S. Tsirkin
2017-03-03 5:40 ` [PATCH v7 kernel 5/5] This patch contains two parts: Wei Wang
2017-03-06 13:23 ` David Hildenbrand
2017-03-09 7:04 ` Wei Wang
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