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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 1EBF5CCFA04 for ; Wed, 5 Nov 2025 07:07:28 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vGXbF-0005xd-Cv; Wed, 05 Nov 2025 02:06:33 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vGXbC-0005xB-7c for qemu-devel@nongnu.org; Wed, 05 Nov 2025 02:06:30 -0500 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 1vGXbA-0004ZP-AL for qemu-devel@nongnu.org; Wed, 05 Nov 2025 02:06:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762326386; 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=Ydvj4qXxZ4ALg2iB228FPFiDHevq43XOtbLOJSolUC4=; b=IqkKlok1ezKddKX+5+fVG0UVfren75LrXBHWDKELjEfmTiodoGsXPRYyitwIHwol2as1o5 3gIs8iu7jEH/36P8MNssdt6sWQ9M4KkaERayCmnLSpANYfS2itqqThovTGsV6LYifOP14p SkmmenEopkSmoDMYtCqQm25ybWJ3Od8= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-619--_t3hOBIP6e5dGdgNmX2zg-1; Wed, 05 Nov 2025 02:06:25 -0500 X-MC-Unique: -_t3hOBIP6e5dGdgNmX2zg-1 X-Mimecast-MFC-AGG-ID: -_t3hOBIP6e5dGdgNmX2zg_1762326384 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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6D74A1800650; Wed, 5 Nov 2025 07:06:24 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.18]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E622519560A2; Wed, 5 Nov 2025 07:06:23 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 43E7D21E6A27; Wed, 05 Nov 2025 08:06:21 +0100 (CET) From: Markus Armbruster To: =?utf-8?Q?Cl=C3=A9ment?= Chigot Cc: Kevin Wolf , qemu-devel@nongnu.org, hreitz@redhat.com, qemu-block@nongnu.org Subject: Re: [PATCH 5/5] vvfat: add support for "size" options In-Reply-To: (=?utf-8?Q?=22Cl=C3=A9ment?= Chigot"'s message of "Fri, 31 Oct 2025 14:07:14 +0100") References: <20250903075721.77623-1-chigot@adacore.com> <20250903075721.77623-6-chigot@adacore.com> <87ms5g3dnq.fsf@pond.sub.org> <87ms5cmtqh.fsf@pond.sub.org> <877bwbcy3e.fsf@pond.sub.org> Date: Wed, 05 Nov 2025 08:06:21 +0100 Message-ID: <878qglot5e.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.788, 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=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Cl=C3=A9ment Chigot writes: > On Fri, Oct 31, 2025 at 12:57=E2=80=AFPM Kevin Wolf wr= ote: [...] >> Imagine the simple case of a qcow2 image file used for the VM. You get >> things stacked like this: >> >> virtio-blk >> | >> v >> qcow2 >> | >> v >> file >> >> You need to open them from bottom to top. Opening a qcow2 image must be >> able to read from the file, so first the file layer must be opened. And >> obvious a virtio-blk device can only use the image after the qcow2 >> layered has been opened. >> >> In your case, this is raw over vvfat. vvfat gets opened first, and then >> raw gets instantiated on top of it. (If you use format=3Dvvfat, then the >> raw layer is left away.) Desirable, because it's simpler. >> Top level options you give to -drive go to the topmost block driver. You >> should be able to still set it on the vvfat level with -drive >> format=3Draw,file.size=3D... Deciding which option goes to which node is >> part of the (rather complicated) bdrv_open() logic in block.c. >> >> What raw does when a size option is given is that it just truncates the >> lower level to the given size. So as vvfat doesn't know the size, it >> still creates a 504 MB image, but raw shows only the first part of it to >> the guest. This results not only in an invalid partition table, but also >> means that as soon as vvfat decides to put a cluster after your limited >> size, you'll see filesystem corruption in the guest. >> >> So your approach to deal with this in vvfat and create a smaller >> filesystem to start with does look right to me. > > Ok thanks for the explanation. It's a bit counter-intuitive that > "size" does not propagate to lower levels, especially if it generates > wrong ones behind the scene. Format "raw" was designed to do nothing, so we have a "do nothing" format for the rigid "format over protocol" system. We've long acquired the means to use a protocol without a format. This made "raw" redundant. I advocate omitting it, because it only complicates matters. Except when you want the one feature "raw" provides beyond "do nothing": carve a slice with options offset and size (commit 2fdc70452a5 "raw_bsd: add offset and size options", 2016). > But IIUC, this would be a much more > complex patch (i.e. changing bdrv_open logic). "size" is for slicing. Can't see how it could simultaneously be forwarded to the next block driver. > Hence, I'm fine keeping this series narrowed to "format=3Dvvfat". > > >> Kevin >>