From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6E1C7606A31A for ; Fri, 30 Oct 2026 17:26:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=Bp5CM1KX Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xMqMk-0006P9-Cv; Fri, 30 Oct 2026 13:26:10 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xMqMh-0006OP-Kh for qemu-devel@nongnu.org; Fri, 30 Oct 2026 13:26:07 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xMqMX-0005Sn-Oz for qemu-devel@nongnu.org; Fri, 30 Oct 2026 13:25:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1793381156; 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: in-reply-to:in-reply-to:references:references; bh=4e24YNhNgAr9GAHFFvSfucjjQCxQNePcq3tV1Oh3ifU=; b=Bp5CM1KXz9fIMRb12bqCox1tQVLRrdal0cbI9drhnq2TTirZiFaMlDvMHBq7REa3ZdLRka 7CZfyvypXFuO3BgXlHMjS9b7VDYNyNmr68dpYiO5ize+kOFI/lCc1E6c6Jp4z39gwqY20t BnmSt760MZtOjZHuH8dI+vRd2nG3OT8= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-369-6-gYWplyNVedz-rwiz8nsg-1; Fri, 30 Oct 2026 13:25:50 -0400 X-MC-Unique: 6-gYWplyNVedz-rwiz8nsg-1 X-Mimecast-MFC-AGG-ID: 6-gYWplyNVedz-rwiz8nsg_1793381148 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 49F071955EAC; Fri, 30 Oct 2026 17:25:47 +0000 (UTC) Received: from redhat.com (unknown [10.44.32.76]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4F1491956087; Fri, 30 Oct 2026 17:25:43 +0000 (UTC) Date: Fri, 30 Oct 2026 18:25:41 +0100 From: Kevin Wolf To: Jean-Louis Dupond Cc: qemu-devel@nongnu.org, Pierrick Bouvier , John Snow , Vladimir Sementsov-Ogievskiy , Hanna Reitz , Fiona Ebner , qemu-block@nongnu.org Subject: Re: [PATCH v2 1/3] block: add bdrv_open_backing_chain_until Message-ID: References: <20260915140444.1223798-1-jean-louis@dupond.be> <20260915140444.1223798-2-jean-louis@dupond.be> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260915140444.1223798-2-jean-louis@dupond.be> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 5 X-Spam_score: 0.5 X-Spam_bar: / X-Spam_report: (0.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 Am 15.09.2026 um 16:04 hat Jean-Louis Dupond geschrieben: > Add a new function called bdrv_open_backing_chain_until. > This is used to open a block up until a specified backing chain. > > Modified bdrv_open_inherit to accept a new arg > 'open_backing_recursively' to force to not open the next backing. > Adjusting all references to keep the existing behaviour. > > Signed-off-by: Jean-Louis Dupond Do you actually have a use case where limiting the backing chain is a requirement? It looks a bit like premature optimisation, because for running a VM, you'll open the whole chain anyway, so it can't be unworkably long. > diff --git a/block.c b/block.c > index f0a6042e61..9763137d8c 100644 > --- a/block.c > +++ b/block.c > @@ -83,6 +83,7 @@ static QLIST_HEAD(, BlockDriver) bdrv_drivers = > static BlockDriverState *bdrv_open_inherit(const char *filename, > const char *reference, > QDict *options, int flags, > + bool open_backing_recursively, > BlockDriverState *parent, > const BdrvChildClass *child_class, > BdrvChildRole child_role, bdrv_open_inherit() already has too many parameters. Can't you just pass BDRV_O_NO_BACKING instead of adding a new one? > @@ -3733,6 +3735,54 @@ free_exit: > return ret; > } > > +int > +bdrv_open_backing_chain_until(BlockDriverState *top_bs, > + const char *base_filename, > + Error **errp) > +{ > + BlockDriverState *curr = top_bs; > + int ret; > + > + GLOBAL_STATE_CODE(); > + > + if (!base_filename) { > + return 0; > + } > + > + if (bdrv_find_backing_image(top_bs, base_filename)) { > + return 0; > + } > + > + while (true) { > + QDict *options = qdict_clone_shallow(curr->options); > + > + ret = bdrv_open_backing_file(curr, options, false, "backing", errp); > + qobject_unref(options); Does this actually do anything? I don't think curr->options can contain "backing" options when you opened it without backing files. It would already fail during the initial open because bdrv_open_backing_file() doesn't absorb the options, so they are considered unknown: $ ./qemu-img info --image-opts driver=qcow2,file.filename=/tmp/overlay.qcow2,backing=foo qemu-img: Could not open 'driver=qcow2,file.filename=/tmp/overlay.qcow2,backing=foo': Block format 'qcow2' does not support the option 'backing' (Using 'qemu-img info' as an example that uses BDRV_O_NO_BACKING.) In your specific 'qemu-img measure' implementation, it seems even impossible to specify backing file options in the BDRV_O_NO_BACKING case in the first place because --image-opts disables the partial loading. Which might actually be another reason not to do the partial opening at all because it's inconsistent behaviour. > + if (ret < 0) { > + return ret; > + } > + > + bdrv_graph_rdlock_main_loop(); > + if (!curr->backing) { > + bdrv_graph_rdunlock_main_loop(); > + error_setg(errp, > + "Did not find '%s' in the backing chain of '%s'", > + base_filename, top_bs->filename); > + return -ENOENT; > + } > + bdrv_graph_rdunlock_main_loop(); > + > + if (bdrv_find_backing_image(curr, base_filename)) { > + return 0; > + } > + > + /* Switch to the next layer */ > + bdrv_graph_rdlock_main_loop(); > + curr = curr->backing->bs; > + bdrv_graph_rdunlock_main_loop(); > + } > +} Kevin