From nobody Thu May 2 08:38:50 2024 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1566389893; cv=none; d=zoho.com; s=zohoarc; b=Is7a/lGZl4t4/fElGW2owJoTG8u0z/ndisb/AVUGgwgJ8L20QU98v3Tsd/PBwoDkOm21WL/V/etbtBka1se1ynK6ihIo0L2fSwjXKHWdsxIm8XVUtnFEAdhuPYwbYm8eyUDYi6S2QQXV94NGSSQgsj8Qz3EoyRPPdC72S1aEMoI= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zoho.com; s=zohoarc; t=1566389893; h=Content-Transfer-Encoding:Cc:Date:From:List-Subscribe:List-Post:List-Id:List-Archive:List-Help:List-Unsubscribe:MIME-Version:Message-ID:Sender:Subject:To:ARC-Authentication-Results; bh=OFAIudO1dWYdQpp+nY6lr760kesRsmGsa28rz/9ctpc=; b=DMCHYH8+0QwgN1NVtAEPMfTxvo5bmu0UtPoGKiWo7phdlOXKc2vonQIUCg2UJBFOeZsKhVk75esWBeXmSqs6EYYwmxtD9/Q8jhIydAljjl/J/wH63HHEGfxPWE6M4sdXV7FSCjlHhNalDWuzWuVlqc5hymwJH8sdf0BFi4MWNO0= ARC-Authentication-Results: i=1; mx.zoho.com; spf=pass (zoho.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail header.from= (p=none dis=none) header.from= Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1566389893274252.6162322196642; Wed, 21 Aug 2019 05:18:13 -0700 (PDT) Received: from localhost ([::1]:47878 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i0PZ4-0004oQ-0k for importer@patchew.org; Wed, 21 Aug 2019 08:18:10 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:34640) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i0PXn-0004FG-7k for qemu-devel@nongnu.org; Wed, 21 Aug 2019 08:16:52 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i0PXl-00045i-8V for qemu-devel@nongnu.org; Wed, 21 Aug 2019 08:16:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56008) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i0PXj-000438-B8 for qemu-devel@nongnu.org; Wed, 21 Aug 2019 08:16:49 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 66FD43083362; Wed, 21 Aug 2019 12:16:45 +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 639012B0E7; Wed, 21 Aug 2019 12:16:28 +0000 (UTC) From: Pankaj Gupta To: qemu-devel@nongnu.org Date: Wed, 21 Aug 2019 17:46:24 +0530 Message-Id: <20190821121624.5382-1-pagupta@redhat.com> MIME-Version: 1.0 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 [10.5.110.44]); Wed, 21 Aug 2019 12:16:45 +0000 (UTC) Content-Transfer-Encoding: quoted-printable 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 v3] 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+importer=patchew.org@nongnu.org Sender: "Qemu-devel" Content-Type: text/plain; charset="utf-8" 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 Reviewed-by: Cornelia Huck --- v3->v3 - Text format fixes - Cornerlia 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..e77881b26f --- /dev/null +++ b/docs/virtio-pmem.rst @@ -0,0 +1,75 @@ + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +QEMU virtio pmem +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + + This document explains the setup and usage of the virtio pmem device + which is available since QEMU v4.1.0. + + The virtio pmem device is a paravirtualized persistent memory device + on regular (i.e non-NVDIMM) storage. + +Usecase +-------- + + Virtio pmem 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=3Dmem1,share,mem-path=3D./virtio_pmem.img= ,size=3D4G + -device virtio-pmem-pci,memdev=3Dmem1,id=3Dnv1 + + where: + - "object memory-backend-file,id=3Dmem1,share,mem-path=3D, size= =3D" + creates a backend file with the specified size. + + - "device virtio-pmem-pci,id=3Dnvdimm1,memdev=3Dmem1" 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 "-objec= t" + 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=3Dmem2,share=3Don,mem-path=3Dvir= tio_pmem2.img,size=3D4G + (qemu) device_add virtio-pmem-pci,id=3Dvirtio_pmem2,memdev=3Dmem2 + +Guest Data Persistence +---------------------- + + Guest data persistence on non-NVDIMM requires guest userspace applications + 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 provid= es + 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. --=20 2.21.0