From patchwork Wed Aug 21 07:36:30 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Pankaj Gupta X-Patchwork-Id: 11105963 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id C2BEA912 for ; Wed, 21 Aug 2019 08:03:15 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A41B922D6D for ; Wed, 21 Aug 2019 08:03:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A41B922D6D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Received: from localhost ([::1]:44820 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i0LaM-0008V1-KB for patchwork-qemu-devel@patchwork.kernel.org; Wed, 21 Aug 2019 04:03:14 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44220) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i0LAy-00084E-AO for qemu-devel@nongnu.org; Wed, 21 Aug 2019 03:37:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i0LAv-0004Dw-Oc for qemu-devel@nongnu.org; Wed, 21 Aug 2019 03:36:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55218) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i0LAu-0004Bj-GA for qemu-devel@nongnu.org; Wed, 21 Aug 2019 03:36:57 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6107D308A98C; Wed, 21 Aug 2019 07:36:53 +0000 (UTC) Received: from dhcp201-121.englab.pnq.redhat.com (ovpn-116-177.sin2.redhat.com [10.67.116.177]) by smtp.corp.redhat.com (Postfix) with ESMTP id BDC9D1001B12; Wed, 21 Aug 2019 07:36:35 +0000 (UTC) From: Pankaj Gupta To: qemu-devel@nongnu.org Date: Wed, 21 Aug 2019 13:06:30 +0530 Message-Id: <20190821073630.2561-1-pagupta@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Wed, 21 Aug 2019 07:36:54 +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: [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: pagupta@redhat.com, david@redhat.com, cohuck@redhat.com, lcapitulino@redhat.com, mst@redhat.com, stefanha@redhat.com, riel@surriel.com, nilal@redhat.com Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" 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 + which is available since QEMU v4.1.0. + + The virtio pmem is a paravirtualized persistent memory device on + regular(i.e non-NVDIMM) storage. + +Usecase +-------- + + Allows to bypass the guest page cache and directly use host page cache. + 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 + 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. + + - "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'." + +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 + 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.