From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48470) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dbqvV-0005Vr-Uo for qemu-devel@nongnu.org; Sun, 30 Jul 2017 12:18:46 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dbqvS-0004GM-Ri for qemu-devel@nongnu.org; Sun, 30 Jul 2017 12:18:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54878) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dbqvS-0004GE-I4 for qemu-devel@nongnu.org; Sun, 30 Jul 2017 12:18:42 -0400 Date: Sun, 30 Jul 2017 19:18:33 +0300 From: "Michael S. Tsirkin" Message-ID: <20170730191735-mutt-send-email-mst@kernel.org> References: <20170713210819-mutt-send-email-mst@kernel.org> <59686EEB.8080805@intel.com> <20170723044036-mutt-send-email-mst@kernel.org> <59781119.8010200@intel.com> <20170726155856-mutt-send-email-mst@kernel.org> <597954E3.2070801@intel.com> <20170729020231-mutt-send-email-mst@kernel.org> <597C83CC.7060702@intel.com> <20170730043922-mutt-send-email-mst@kernel.org> <286AC319A985734F985F78AFA26841F739288D85@shsmsx102.ccr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <286AC319A985734F985F78AFA26841F739288D85@shsmsx102.ccr.corp.intel.com> Subject: Re: [Qemu-devel] [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Wang, Wei W" Cc: "linux-kernel@vger.kernel.org" , "qemu-devel@nongnu.org" , "virtualization@lists.linux-foundation.org" , "kvm@vger.kernel.org" , "linux-mm@kvack.org" , "david@redhat.com" , "cornelia.huck@de.ibm.com" , "akpm@linux-foundation.org" , "mgorman@techsingularity.net" , "aarcange@redhat.com" , "amit.shah@redhat.com" , "pbonzini@redhat.com" , "liliang.opensource@gmail.com" , "virtio-dev@lists.oasis-open.org" , "yang.zhang.wz@gmail.com" , "quan.xu@aliyun.com" On Sun, Jul 30, 2017 at 05:59:17AM +0000, Wang, Wei W wrote: > On Sunday, July 30, 2017 12:23 PM, Michael S. Tsirkin wrote: > > On Sat, Jul 29, 2017 at 08:47:08PM +0800, Wei Wang wrote: > > > On 07/29/2017 07:08 AM, Michael S. Tsirkin wrote: > > > > On Thu, Jul 27, 2017 at 10:50:11AM +0800, Wei Wang wrote: > > > > > > > > OK I thought this over. While we might need these new APIs > > > > > > > > in the future, I think that at the moment, there's a way to > > > > > > > > implement this feature that is significantly simpler. Just > > > > > > > > add each s/g as a separate input buffer. > > > > > > > Should it be an output buffer? > > > > > > Hypervisor overwrites these pages with zeroes. Therefore it is > > > > > > writeable by device: DMA_FROM_DEVICE. > > > > > Why would the hypervisor need to zero the buffer? > > > > The page is supplied to hypervisor and can lose the value that is > > > > there. That is the definition of writeable by device. > > > > > > I think for the free pages, it should be clear that they will be added > > > as output buffer to the device, because (as we discussed) they are > > > just hints, and some of them may be used by the guest after the report_ API is > > invoked. > > > The device/hypervisor should not use or discard them. > > > > Discarding contents is exactly what you propose doing if migration is going on, > > isn't it? > > That's actually a different concept. Please let me explain it with this example: > > The hypervisor receives the hint saying the guest PageX is a free page, but as we know, > after that report_ API exits, the guest kernel may take PageX to use, so PageX is not free > page any more. At this time, if the hypervisor writes to the page, that would crash the guest. > So, I think the cornerstone of this work is that the hypervisor should not touch the > reported pages. > > Best, > Wei That's a hypervisor implementation detail. From guest point of view, discarding contents can not be distinguished from writing old contents.