From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1549C433E0 for ; Fri, 3 Jul 2020 17:23:44 +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 ADA48208FE for ; Fri, 3 Jul 2020 17:23:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="TAaELTfu" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ADA48208FE 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+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:47652 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jrPPb-0001zt-Ud for qemu-devel@archiver.kernel.org; Fri, 03 Jul 2020 13:23:43 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:57144) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jrPOZ-0000RG-RP for qemu-devel@nongnu.org; Fri, 03 Jul 2020 13:22:39 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:45835 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jrPOX-00041k-DE for qemu-devel@nongnu.org; Fri, 03 Jul 2020 13:22:39 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1593796956; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=oMLOddMKbM8xSJOMR6at8tUdRCl0ib/izdNU0GCrm/c=; b=TAaELTfuLJekZRN2WvN7+f5pAhMyQI0OoZ0cWB1ShBSoEQMJNOyik2UwH5GKd0tIK9mSQw X7n/PVQAbL+3mS+0bg/YnNOGMQrWffyyCa3lAiwkyzcDTpVEn25Q3FNYk5KjGZNVowEwC7 bzmSx8U8IrNmDLDG/6B01Tk/6moo0JQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-216-L9tpO6V7NFi8WVxDSUwcWA-1; Fri, 03 Jul 2020 13:22:32 -0400 X-MC-Unique: L9tpO6V7NFi8WVxDSUwcWA-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 79BA3107ACF2; Fri, 3 Jul 2020 17:22:31 +0000 (UTC) Received: from redhat.com (unknown [10.36.110.57]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CF3D47BD44; Fri, 3 Jul 2020 17:22:27 +0000 (UTC) Date: Fri, 3 Jul 2020 18:22:24 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: "Denis V. Lunev" Subject: Re: [PATCH 0/6] migration: bring savevm/loadvm/delvm over to QMP Message-ID: <20200703172224.GT2213227@redhat.com> References: <20200702175754.2211821-1-berrange@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.14.3 (2020-06-14) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/03 03:17:33 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , Peter Krempa , qemu-block@nongnu.org, Juan Quintela , Markus Armbruster , qemu-devel@nongnu.org, Pavel Dovgalyuk , Paolo Bonzini , Max Reitz , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jul 03, 2020 at 08:15:44PM +0300, Denis V. Lunev wrote: > On 7/2/20 8:57 PM, Daniel P. Berrangé wrote: > > When QMP was first introduced some 10+ years ago now, the snapshot > > related commands (savevm/loadvm/delvm) were not converted. This was > > primarily because their implementation causes blocking of the thread > > running the monitor commands. This was (and still is) considered > > undesirable behaviour both in HMP and QMP. > > > > In theory someone was supposed to fix this flaw at some point in the > > past 10 years and bring them into the QMP world. Sadly, thus far it > > hasn't happened as people always had more important things to work > > on. Enterprise apps were much more interested in external snapshots > > than internal snapshots as they have many more features. > > > > Meanwhile users still want to use internal snapshots as there is > > a certainly simplicity in having everything self-contained in one > > image, even though it has limitations. Thus the apps that end up > > executing the savevm/loadvm/delvm via the "human-monitor-command" > > QMP command. > > > > > > IOW, the problematic blocking behaviour that was one of the reasons > > for not having savevm/loadvm/delvm in QMP is experienced by applications > > regardless. By not portting the commands to QMP due to one design flaw, > > we've forced apps and users to suffer from other design flaws of HMP ( > > bad error reporting, strong type checking of args, no introspection) for > > an additional 10 years. This feels rather sub-optimal :-( > > > > In practice users don't appear to care strongly about the fact that these > > commands block the VM while they run. I might have seen one bug report > > about it, but it certainly isn't something that comes up as a frequent > > topic except among us QEMU maintainers. Users do care about having > > access to the snapshot feature. > > > > Where I am seeing frequent complaints is wrt the use of OVMF combined > > with snapshots which has some serious pain points. This is getting worse > > as the push to ditch legacy BIOS in favour of UEFI gain momentum both > > across OS vendors and mgmt apps. Solving it requires new parameters to > > the commands, but doing this in HMP is super unappealing. > > > > > > > > After 10 years, I think it is time for us to be a little pragmatic about > > our handling of snapshots commands. My desire is that libvirt should never > > use "human-monitor-command" under any circumstances, because of the > > inherant flaws in HMP as a protocol for machine consumption. If there > > are flaws in QMP commands that's fine. If we fix them in future, we can > > deprecate the current QMP commands and remove them not too long after, > > without being locked in forever. > > > > > > Thus in this series I'm proposing a direct 1-1 mapping of the existing > > HMP commands for savevm/loadvm/delvm into QMP as a first step. This does > > not solve the blocking thread problem, but it does eliminate the error > > reporting, type checking and introspection problems inherant to HMP. > > We're winning on 3 out of the 4 long term problems. > > > > If someone can suggest a easy way to fix the thread blocking problem > > too, I'd be interested to hear it. If it involves a major refactoring > > then I think user are better served by unlocking what look like easy > > wins today. > > > > With a QMP variant, we reasonably deal with the problems related to OVMF: > > > > - The logic to pick which disk to store the vmstate in is not > > satsifactory. > > > > The first block driver state cannot be assumed to be the root disk > > image, it might be OVMF varstore and we don't want to store vmstate > > in there. > > > > - The logic to decide which disks must be snapshotted is hardwired > > to all disks which are writable > > > > Again with OVMF there might be a writable varstore, but this can be > > raw rather than qcow2 format, and thus unable to be snapshotted. > > While users might wish to snapshot their varstore, in some/many/most > > cases it is entirely uneccessary. Users are blocked from snapshotting > > their VM though due to this varstore. > > > > These are solved by adding two parameters to the commands. The first is > > a block device node name that identifies the image to store vmstate in, > > and the second is a list of node names to exclude from snapshots. > > > > In the block code I've only dealt with node names for block devices, as > > IIUC, this is all that libvirt should need in the -blockdev world it now > > lives in. IOW, I've made not attempt to cope with people wanting to use > > these QMP commands in combination with -drive args. > > > > I've done some minimal work in libvirt to start to make use of the new > > commands to validate their functionality, but this isn't finished yet. > > > > My ultimate goal is to make the GNOME Boxes maintainer happy again by > > having internal snapshots work with OVMF: > > > > https://gitlab.gnome.org/GNOME/gnome-boxes/-/commit/c486da262f6566326fbcb5e= > > f45c5f64048f16a6e > > > > Daniel P. Berrang=C3=A9 (6): > > migration: improve error reporting of block driver state name > > migration: introduce savevm, loadvm, delvm QMP commands > > block: add ability to filter out blockdevs during snapshot > > block: allow specifying name of block device for vmstate storage > > migration: support excluding block devs in QMP snapshot commands > > migration: support picking vmstate disk in QMP snapshot commands > > > > block/monitor/block-hmp-cmds.c | 4 +- > > block/snapshot.c | 68 +++++++++++++++++++------ > > include/block/snapshot.h | 21 +++++--- > > include/migration/snapshot.h | 10 +++- > > migration/savevm.c | 71 +++++++++++++++++++------- > > monitor/hmp-cmds.c | 20 ++------ > > qapi/migration.json | 91 ++++++++++++++++++++++++++++++++++ > > replay/replay-snapshot.c | 4 +- > > softmmu/vl.c | 2 +- > > 9 files changed, 228 insertions(+), 63 deletions(-) > > I have tried to work in this interface in 2016. That time > we have struggled with the idea that this QMP interface should > be ready to work asynchronously. > > Write-protect userfaultfd was merged into vanilla Linux > thus it is time to async savevm interface, which will also > bring async loadvm and some rework for state storing. > > Thus I think that with the introduction of the QMP interface > we should at least run save VM not from the main > thread but from the background with the event at the end. spawning a thread in which to invoke save_snapshot() and load_snapshot() is easy enough. I'm not at all clear on what we need in the way of mutex locking though, to make those methods safe to run in a thread that isn't the main event loop. Even with savevm/loadvm being blocking, we could introduce a QMP event straight away, and document that users shouldn't assume the operation is complete until they see the event. That would let us make the commands non-blocking later with same documented semantics. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|