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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,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 79E9CC433ED for ; Tue, 18 May 2021 16:53:24 +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 A7F18610CC for ; Tue, 18 May 2021 16:53:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A7F18610CC 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]:56966 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lj2yA-0006CQ-Mn for qemu-devel@archiver.kernel.org; Tue, 18 May 2021 12:53:22 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56190) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lj2vu-0004DJ-Nq for qemu-devel@nongnu.org; Tue, 18 May 2021 12:51:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:33993) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lj2vr-0005xP-Dr for qemu-devel@nongnu.org; Tue, 18 May 2021 12:51:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621356658; h=from:from: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=dQGswRwxWt51vx5UQqvMgViazo3LFbiANBgR0pP24pw=; b=cLtoPuDmNSuyE5nn8HlLK7lRHrr5camqhJ4pq/eGpQJRPR+9tKNCjqTfvMV1Hr72EwEXOZ 0Z3K3fqH/LdsbbCpodjLDXu29vSSQsTTIqZ9Fj9rUulvvo6jwkkBxIH2ztWrp503OJUZz/ pPXaJg454zrUOLqcn7P/VbXZ5klkf0Y= 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-483-yJFOBfisNnmrffK8-eIJzg-1; Tue, 18 May 2021 12:50:55 -0400 X-MC-Unique: yJFOBfisNnmrffK8-eIJzg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DF97910CE788; Tue, 18 May 2021 16:50:40 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-115-121.ams2.redhat.com [10.36.115.121]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9457C69322; Tue, 18 May 2021 16:49:58 +0000 (UTC) Subject: Re: Qemu block filter insertion/removal API To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: From: Max Reitz Message-ID: <0f0ede72-5b40-4896-a9c4-488b31e74d5f@redhat.com> Date: Tue, 18 May 2021 18:49:56 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.374, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no 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: , Cc: Kevin Wolf , Peter Krempa , "libvir-list@redhat.com" , Dmitry Mishin , Igor Sukhih , qemu-devel , yur@virtuozzo.com, Nikolay Shirokovskiy , Stefan Hajnoczi , "Denis V. Lunev" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 17.05.21 14:44, Vladimir Sementsov-Ogievskiy wrote: > Hi all! > > I'd like to be sure that we know where we are going to. > > In blockdev-era where qemu user is aware about block nodes, all nodes > have good names and controlled by user we can efficiently use block > filters. > > We already have some useful filters: copy-on-read, throttling, compress. > In my parallel series I make backup-top filter public and useful without > backup block jobs. But now filters could be inserted only together with > opening their child. We can specify filters in qemu cmdline, or filter > can take place in the block node chain created by blockdev-add. > > Still, it would be good to insert/remove filters on demand. > > Currently we are going to use x-blockdev-reopen for this. Still it can't > be used to insert a filter above root node (as x-blockdev-reopen can > change only block node options and their children). In my series "[PATCH > 00/21] block: publish backup-top filter" I propose (as Kevin suggested) > to modify qom-set, so that it can set drive option of running device. > That's not difficult, but it means that we have different scenario of > inserting/removing filters: > > 1. filter above root node X: > > inserting: > >   - do blockdev-add to add a filter (and specify X as its child) >   - do qom-set to set new filter as a rood node instead of X > > removing > >   - do qom-set to make X a root node again >   - do blockdev-del to drop a filter > > 2. filter between two block nodes P and X. (For example, X is a backing > child of P) > > inserting > >   - do blockdev-add to add a filter (and specify X as its child) >   - do blockdev-reopen to set P.backing = filter > > remvoing > >   - do blockdev-reopen to set P.backing = X >   - do blockdev-del to drop a filter > > > And, probably we'll want transaction support for all these things. > > > Is it OK? Or do we need some kind of additional blockdev-replace > command, that can replace one node by another, so in both cases we will do > > inserting: > >   - blockdev-add filter >   - blockdev-replace (make all parents of X to point to the new filter > instead (except for the filter itself of course) > > removing >   - blockdev-replace (make all parante of filter to be parents of X > instead) >   - blockdev-del filter > > > It's simple to implement, and it seems for me that it is simpler to use. > Any thoughts? I’m afraid as a non-user of the blockdev interface, I can’t give a valuable opinion that would have some actual weight. Doesn’t stop me from giving my personal and potentially invaluable opinion, though, obviously: I think we expect all users to know the block graph, so they should be able to distinguish between cases 1 and 2. However, I can imagine having to distinguish still is kind of a pain, especially if it were trivial for qemu to let the user not having to worry about it at all. Also, if you want a filter unconditionally above some node, all the qom-set and blockdev-reopen operations for all of the original node’s parents would need to happen atomically. As you say, those operations should perhaps be transactionable anyway, but... Implementing blockdev-replace would provide this for much less cost now, I suppose? I guess it can be argued that the downside is that having blockdev-replace means less pressure to make qom-set for drive and blockdev-reopen transactionable. But well. I don’t really have anything against a blockdev-replace, but again, I don’t know whether my opinion on this topic really has weight. Max