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Wed, 21 Aug 2019 11:47:10 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AF08B2E07C; Wed, 21 Aug 2019 11:47:10 +0000 (UTC) Received: from zmail21.collab.prod.int.phx2.redhat.com (zmail21.collab.prod.int.phx2.redhat.com [10.5.83.24]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 9E5791803B58; Wed, 21 Aug 2019 11:47:10 +0000 (UTC) Date: Wed, 21 Aug 2019 07:47:10 -0400 (EDT) From: Pankaj Gupta To: Cornelia Huck Message-ID: <1761975250.9673246.1566388030011.JavaMail.zimbra@redhat.com> In-Reply-To: <20190821132440.066dfbba.cohuck@redhat.com> References: <20190821073630.2561-1-pagupta@redhat.com> <20190821132440.066dfbba.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [10.67.116.177, 10.4.195.26] Thread-Topic: virtio pmem: user document Thread-Index: ++MfrA1XY/bimkRUBv5J22FkR4vFWQ== X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Wed, 21 Aug 2019 11:47:10 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH v2] virtio pmem: user document X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mst@redhat.com, david@redhat.com, qemu-devel@nongnu.org, lcapitulino@redhat.com, stefanha@redhat.com, riel@surriel.com, nilal@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi Cornelia, > > This patch documents the steps to use virtio pmem. > > It also documents other useful information about > > virtio pmem e.g use-case, comparison with Qemu NVDIMM > > backend and current limitations. > > > > Signed-off-by: Pankaj Gupta > > --- > > v1->v2 > > - Fixes on text format and 'Guest Data persistence' > > section - Cornelia > > > > docs/virtio-pmem.rst | 75 ++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 75 insertions(+) > > create mode 100644 docs/virtio-pmem.rst > > > > diff --git a/docs/virtio-pmem.rst b/docs/virtio-pmem.rst > > new file mode 100644 > > index 0000000000..0346e61674 > > --- /dev/null > > +++ b/docs/virtio-pmem.rst > > @@ -0,0 +1,75 @@ > > + > > +======================== > > +QEMU virtio pmem > > +======================== > > + > > + This document explains the setup and usage of virtio pmem device > > s/virtio pmem device/the virtio pmem device/ done > > > + which is available since QEMU v4.1.0. > > + > > + The virtio pmem is a paravirtualized persistent memory device on > > s/The virtio pmem/The virtio pmem device/ o.k > > > + regular(i.e non-NVDIMM) storage. > > missing blank before '(' sure > > > + > > +Usecase > > +-------- > > + > > + Allows to bypass the guest page cache and directly use host page cache. > > "Virtio pmem allows to..." ? done. > > > + This reduces guest memory footprint as the host can make efficient > > + memory reclaim decisions under memory pressure. > > + > > +o How does virtio-pmem compare to the nvdimm emulation supported by QEMU? > > + > > + NVDIMM emulation on regular(i.e. non-NVDIMM) host storage does not > > missing blank before '(' done. > > > + persist the guest writes as there are no defined semantics in the device > > + specification. The virtio pmem device provides guest write persistence > > + on non-NVDIMM host storage. > > + > > +virtio pmem usage > > +----------------- > > + > > + A virtio pmem device backed by a memory-backend-file can be created on > > + the QEMU command line as in the following example: > > + > > + -object > > memory-backend-file,id=mem1,share,mem-path=./virtio_pmem.img,size=4G > > + -device virtio-pmem-pci,memdev=mem1,id=nv1 > > + > > + where: > > + - "object memory-backend-file,id=mem1,share,mem-path=, > > size=" > > + creates a backend file of size on a mem-path. > > "a backend file with the specified size" ? done. > > > + > > + - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem > > + pci device whose storage is provided by above memory backend device. > > + > > + Multiple virtio pmem devices can be created if multiple pairs of > > "-object" > > + and "-device" are provided. > > + > > +Hotplug > > +------- > > + > > +"Virtio pmem devices can be hotplugged via the QEMU monitor. First, the > > +memory backing has to be added via 'object_add'; afterwards, the virtio > > +pmem device can be added via 'device_add'." > > Please lose the '"' (copy/paste leftover, I presume? :) Done :) > > > + > > +For example, the following commands add another 4GB virtio pmem device to > > +the guest: > > + > > + (qemu) object_add > > memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G > > + (qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2 > > + > > +Guest Data Persistence > > +---------------------- > > + > > + Guest data persistence on non-NVDIMM requires guest userspace application > > to > > s/application/applications/ ? done. > > > + perform fsync/msync. This is different from a real nvdimm backend where > > no > > + additional fsync/msync is required. This is to persist guest writes in > > host > > + backing file which otherwise remains in host page cache and there is risk > > of > > + losing the data in case of power failure. > > + > > + With virtio pmem device, MAP_SYNC mmap flag is not supported. This > > provides > > + a hint to application to perform fsync for write persistence. > > + > > +Limitations > > +------------ > > +- Real nvdimm device backend is not supported. > > +- virtio pmem hotunplug is not supported. > > +- ACPI NVDIMM features like regions/namespaces are not supported. > > +- ndctl command is not supported. > > Only some nits from my side, otherwise looks good to me. Thank you for the review. Will post a v3 with the changes. Best regards, Pankaj >