From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39080) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cmr2f-0000JO-00 for qemu-devel@nongnu.org; Sat, 11 Mar 2017 19:07:21 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cmr2b-0004f1-Jk for qemu-devel@nongnu.org; Sat, 11 Mar 2017 19:07:20 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53836) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cmr2b-0004eT-BH for qemu-devel@nongnu.org; Sat, 11 Mar 2017 19:07:17 -0500 Date: Sun, 12 Mar 2017 02:07:14 +0200 From: "Michael S. Tsirkin" Message-ID: <20170312020638-mutt-send-email-mst@kernel.org> References: <1488519630-89058-1-git-send-email-wei.w.wang@intel.com> <1488519630-89058-4-git-send-email-wei.w.wang@intel.com> <20170309141411.GZ16328@bombadil.infradead.org> <58C28FF8.5040403@intel.com> <20170310175349-mutt-send-email-mst@kernel.org> <20170310171143.GA16328@bombadil.infradead.org> <58C3E6A3.1000000@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <58C3E6A3.1000000@intel.com> Subject: Re: [Qemu-devel] [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Wei Wang Cc: Matthew Wilcox , 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 , Paolo Bonzini , Cornelia Huck , Amit Shah , Dave Hansen , Andrea Arcangeli , David Hildenbrand , Liang Li On Sat, Mar 11, 2017 at 07:59:31PM +0800, Wei Wang wrote: > On 03/11/2017 01:11 AM, 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; > > } > > > > (note this is a single page of order N, so we leave the page count bits > > set to 0, meaning one page). > > > > I'm thinking what if the guest needs to transfer these much physically > continuous > memory to host: 1GB+2MB+64KB+32KB+16KB+4KB. > Is it going to use Six 64-bit chunks? Would it be simpler if we just > use the 128-bit chunk format (we can drop the previous normal 64-bit > format)? > > Best, > Wei I think I prefer that as a more straightforward approach, but I can live with either approach. -- MST