From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57625) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gZyyR-0002O0-Mq for qemu-devel@nongnu.org; Thu, 20 Dec 2018 09:06:52 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gZyyO-0001K0-J7 for qemu-devel@nongnu.org; Thu, 20 Dec 2018 09:06:51 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51118) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gZyyO-0001JC-Aj for qemu-devel@nongnu.org; Thu, 20 Dec 2018 09:06:48 -0500 Date: Thu, 20 Dec 2018 09:06:41 -0500 From: "Michael S. Tsirkin" Message-ID: <20181220090435-mutt-send-email-mst@kernel.org> References: <5eb87eb9a389242772375dd52bd97c17564a0829.1545120100.git.yi.z.zhang@linux.intel.com> <20181218085249-mutt-send-email-mst@kernel.org> <20181219091017.GA35378@tiger-server> <20181219105555-mutt-send-email-mst@kernel.org> <20181220030312.GA70591@tiger-server> <20181219223700-mutt-send-email-mst@kernel.org> <20181220053739.GB70591@tiger-server> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181220053739.GB70591@tiger-server> Subject: Re: [Qemu-devel] [PATCH V7 6/6] hostmem-file: add 'sync' option List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: peter.maydell@linaro.org, yu.c.zhang@linux.intel.com, ehabkost@redhat.com, imammedo@redhat.com, qemu-devel@nongnu.org, pbonzini@redhat.com, stefanha@redhat.com, crosthwaite.peter@gmail.com, richard.henderson@linaro.org, xiaoguangrong.eric@gmail.com On Thu, Dec 20, 2018 at 01:37:40PM +0800, Yi Zhang wrote: > On 2018-12-19 at 22:42:07 -0500, Michael S. Tsirkin wrote: > > On Thu, Dec 20, 2018 at 11:03:12AM +0800, Yi Zhang wrote: > > > On 2018-12-19 at 10:59:10 -0500, Michael S. Tsirkin wrote: > > > > On Wed, Dec 19, 2018 at 05:10:18PM +0800, Yi Zhang wrote: > > > > > > > + > > > > > > > + - 'sync' option of memory-backend-file is not 'off', and > > > > > > > + > > > > > > > + - 'share' option of memory-backend-file is 'on'. > > > > > > > + > > > > > > > + - 'pmem' option of memory-backend-file is 'on' > > > > > > > + > > > > > > > > > > > > Wait isn't this what pmem was supposed to do? > > > > > > Doesn't it mean "persistent memory"? > > > > > pmem is a option for memory-backend-file, user should know the backend > > > > > is in host persistent memory, with this flags on, while there is a host crash > > > > > or a power failures. > > > > > > > > > > [1] Qemu will take necessary operations to guarantee the persistence. > > > > > https://patchwork.ozlabs.org/cover/944749/ > > > > > > > > > > [2] Host kernel also take opretions to consistent filesystem metadata. > > > > > Add MAP_SYNC flags. > > > > > > > > OK so I'm a user. Can you educate me please? > > > We suppose an administrator should know it, what is the back-end region coming from, > > > is it persistent? what is the font-end device is? a volatile dimm or an > > > nonvolatile dimm? then they can choice put the pmem=on[off] and sync=on[off]. > > > If he didn't, we encourage that don't set these 2 flags. > > > > > > > When should MAP_SYNC not > > > > be set? Are there any disadvantages (e.g. performance?)? > > > Not only the performance, sometimes like the front-end device is an > > > volatile ram, we don't wanna set such option although the backend is a > > > novolatile memory, if power lose, all of thing should lose in this ram. > > > > > > > > I am not sure how does above answer the questions. If I don't know, > > neither will the hypothetical administrator. Looks like a better > > interface is needed to make the choice on behalf of the user. > > > By default, we have already ignore the 2 flags, unless the administrator > know how to make that use. By-now, seems we don't have a better way to detect what > memory-backend-file is, a persistent memory or not. In that case how about an interface where user tells QEMU "this backend is in persistent memory"? > > > > > > > > > > -- > > > > MST > > > > > >