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 1983ECCF9F8 for ; Fri, 7 Nov 2025 08:07:13 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vHHUW-0006PX-Mq; Fri, 07 Nov 2025 03:06:40 -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 1vHHUV-0006P3-40 for qemu-devel@nongnu.org; Fri, 07 Nov 2025 03:06:39 -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 1vHHUS-0001W0-Tt for qemu-devel@nongnu.org; Fri, 07 Nov 2025 03:06:38 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762502796; 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=OMPTlux4GQiWlk+tfespG8VCNX1mSidRe58c2R+4YT4=; b=dTbHLUSYG3edXbyDHTfelvEFiL2TPxkjglNXtU5/hZIZ1krdo/N+wvM+d+Xv3gxF3JQuWw REUzQUm/GsSEt0Ta1tRC1XHV3IevA5meckYDx+2huJRodyjnlckrWnpwfNFV8PGOKGuENX 3q/7fXJR5MiveDgQ2RP2CUW/F+Tu/DQ= 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-199-I0jSD1kRPuyqVurHKGmO8g-1; Fri, 07 Nov 2025 03:06:32 -0500 X-MC-Unique: I0jSD1kRPuyqVurHKGmO8g-1 X-Mimecast-MFC-AGG-ID: I0jSD1kRPuyqVurHKGmO8g_1762502791 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 5140C1800673; Fri, 7 Nov 2025 08:06:31 +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 B2C7D19560A7; Fri, 7 Nov 2025 08:06:30 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 24C9F21E6A27; Fri, 07 Nov 2025 09:06:28 +0100 (CET) From: Markus Armbruster To: Kevin Wolf Cc: =?utf-8?Q?Cl=C3=A9ment?= Chigot , 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: (Kevin Wolf's message of "Wed, 5 Nov 2025 10:35:58 +0100") References: <20250903075721.77623-1-chigot@adacore.com> <20250903075721.77623-6-chigot@adacore.com> <874ir9ot1a.fsf@pond.sub.org> Date: Fri, 07 Nov 2025 09:06:28 +0100 Message-ID: <875xbme06z.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: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.271, 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_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-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: 2.1.29 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 Kevin Wolf writes: > Am 05.11.2025 um 08:08 hat Markus Armbruster geschrieben: >> Kevin Wolf writes: >>=20 >> [...] >>=20 >> > To me it looks a bit like what we really want is an enum for floppy >> > sizes (though is there any real reason why we have only those two?), b= ut >> > an arbitrary size for hard disks. >> > >> > Without the enum, obviously, users could specify 1440k and that would = do >> > the right thing. Maybe special casing whatever 1.44M and 2.88M result >> > in and translating them into 1440k and 2880k could be more justifiable >> > than special casing 1M and 2M, but it would still be ugly. >> > >> > Markus, do you have any advice how this should be represented in QAPI? >>=20 >> Still want answers here? > > Yes, I'm still not sure how we could best represent both hard disk and > floppy sizes in vvfat in a way that isn't completely counterintuitive > for users, that also isn't just arbitrary magic and that works on the > command line. > > Unless the need for different sizes has gone away, but I don't think we > found any other solution for the problem that would not require a > configurable disk/file system size? Let me recap the problem. Please correct my misunderstandings, if any. Hard disks can have almost arbitrary sizes. Almost, because it still needs to be a multiple of the block size. Floppy disks have one of a small set of well-known sizes. I vaguely recall that we generally derive the device's actual size from the backend's size. Some devices reject certain sizes. For instance, SD cards require a power of 2. Most devices seem to accept anything. I can create an IDE, SCSI, or floppy disk backed by a raw image of one byte. I have no idea how it would behave. As is, the vvfat backend can only do certain sizes, configurable with parameters @floppy and @fat-type. They work for floppies, but not for SD cards, since they're not powers of two. Instead of deriving size and CHS from @floppy and @fat-type, Cl=C3=A9ment proposes to specify the size (and derive fat-type and CHS[*]?). In QMP, we specify the size in bytes. This is fine regardless of size; management applications don't mind sending things like "size": 1474560. In HMP and the command line, big byte sizes are inconvenient. That's why we support suffixes there. size=3D256M is a fine way to pick an SD card's size. The size suffixes seem inconvenient for floppies, though. For instance, 2 heads * 80 tracks * 18 sectors * 512 bytes =3D 1474560 bytes =3D 1440 KiBytes, but size=3D1.44M does not work: 1.44 MiBytes =3D 1509949.44 Bytes. However, size=3D1440K does. This leads me to suggest to simply stick to numeric size, and use appropriate suffixes. These are obvious enough for anything but floppies. So advise users "use K for floppies"[**]. If this isn't good enough, I can help you explore fancier parts of QAPI, such as alternate types. [*] I guess we could support specifying an optional fat-type in addition to size, and derive only CHS then. [**] Even for a hypothetical floppy with an odd number of 512 byte sectors: .5K works, because .5 * 1024 is an integer.