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Date: Fri, 10 Mar 2017 13:18:25 -0800
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of
 VIRTIO_BALLOON_F_CHUNK_TRANSFER
Message-ID: <20170310211825.GB16328@bombadil.infradead.org>
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To: "Michael S. Tsirkin" <mst@redhat.com>
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>

On Fri, Mar 10, 2017 at 09:10:53PM +0200, Michael S. Tsirkin wrote:
> > 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
> 
> huge page sizes go up to gigabytes.

There was supposed to be a '...' there.  For a 16GB hugepage (largest
size I know of today), that'd be:

bits 0-21 set, 22 clear, 23-33 page count, 34-63 PFN, page size 16G

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