From patchwork Tue Jul 30 06:46:58 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Pankaj Gupta X-Patchwork-Id: 1138796 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 ozlabs.org (Postfix) with ESMTPS id 45yRvT3pGxz9sBF for ; Tue, 30 Jul 2019 16:47:48 +1000 (AEST) Received: from localhost ([::1]:58334 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hsLvG-0008Gq-RU for incoming@patchwork.ozlabs.org; Tue, 30 Jul 2019 02:47:46 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:33021) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hsLuy-0008E3-MG for qemu-devel@nongnu.org; Tue, 30 Jul 2019 02:47:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hsLux-0008Pp-K3 for qemu-devel@nongnu.org; Tue, 30 Jul 2019 02:47:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46216) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hsLux-0008P9-DO for qemu-devel@nongnu.org; Tue, 30 Jul 2019 02:47:27 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id BDCDAC024AED; Tue, 30 Jul 2019 06:47:26 +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 6BBC6600CD; Tue, 30 Jul 2019 06:47:21 +0000 (UTC) From: Pankaj Gupta To: qemu-devel@nongnu.org Date: Tue, 30 Jul 2019 12:16:58 +0530 Message-Id: <20190730064658.27369-2-pagupta@redhat.com> In-Reply-To: <20190730064658.27369-1-pagupta@redhat.com> References: <20190730064658.27369-1-pagupta@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Tue, 30 Jul 2019 06:47:26 +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] 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: aarcange@redhat.com, pagupta@redhat.com, cohuck@redhat.com, david@redhat.com, mst@redhat.com, lcapitulino@redhat.com, stefanha@redhat.com, riel@surriel.com, nilal@redhat.com Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.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 --- docs/virtio-pmem.txt | 65 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) create mode 100644 docs/virtio-pmem.txt diff --git a/docs/virtio-pmem.txt b/docs/virtio-pmem.txt new file mode 100644 index 0000000000..fc61eebb20 --- /dev/null +++ b/docs/virtio-pmem.txt @@ -0,0 +1,65 @@ + +QEMU virtio pmem +=================== + + This document explains the usage of virtio pmem device + which is available since QEMU v4.1.0. + + The virtio pmem is paravirtualized persistent memory device + on regular(non-NVDIMM) storage. + +Usecase +-------- + Allows to bypass the guest page cache and directly use host page cache. + This reduces guest memory footprint as 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(non-NVDIMM) host storage does not persists + the guest writes as there are no defined semantecs in the device specification. + With virtio pmem device, guest write persistence on non-NVDIMM storage is + supported. + +virtio pmem usage +----------------- + virtio pmem device is created with a memory-backend-file with the below + options: + + -machine pc -m 8G,slots=$N,maxmem=$MAX_SIZE + -object memory-backend-file,id=mem1,share,mem-path=$PATH,size=$SIZE + -device virtio-pmem-pci,memdev=mem1,id=nv1 + + where: + - "object memory-backend-file,id=mem1,share,mem-path=$PATH,size=$VIRTIO_PMEM_SIZE" + creates a backend storage of size $SIZE on a file $PATH. All + accesses to the virtio pmem device go to the file $PATH. + + - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem + 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 +------- +Accomplished by two monitor commands "object_add" and "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 than real nvdimm backend where no additional +fsync/msync is required for data 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.