From patchwork Fri Jan 25 23:48:37 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Blake X-Patchwork-Id: 1031354 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=fail (p=none dis=none) header.from=redhat.com Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 43mbMv6y6gz9sBn for ; Sat, 26 Jan 2019 10:49:15 +1100 (AEDT) Received: from localhost ([127.0.0.1]:52559 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gnBDl-0000n4-Vs for incoming@patchwork.ozlabs.org; Fri, 25 Jan 2019 18:49:14 -0500 Received: from eggs.gnu.org ([209.51.188.92]:43833) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gnBDR-0000kV-TT for qemu-devel@nongnu.org; Fri, 25 Jan 2019 18:48:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gnBDQ-0000AG-0B for qemu-devel@nongnu.org; Fri, 25 Jan 2019 18:48:53 -0500 Received: from mx1.redhat.com ([209.132.183.28]:32846) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gnBDI-0008SH-6J; Fri, 25 Jan 2019 18:48:46 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8992C7BDA1; Fri, 25 Jan 2019 23:48:41 +0000 (UTC) Received: from blue.redhat.com (ovpn-117-44.phx2.redhat.com [10.3.117.44]) by smtp.corp.redhat.com (Postfix) with ESMTP id E62EF101E5B1; Fri, 25 Jan 2019 23:48:38 +0000 (UTC) From: Eric Blake To: qemu-devel@nongnu.org Date: Fri, 25 Jan 2019 17:48:37 -0600 Message-Id: <20190125234837.2272-1-eblake@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Fri, 25 Jan 2019 23:48:41 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH v2] qemu-nbd: Deprecate qemu-nbd --partition X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "reviewer:Incompatible changes" , thuth@redhat.com, vsementsov@virtuozzo.com, rjones@redhat.com, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" The existing qemu-nbd --partition code claims to handle logical partitions up to 8, since its introduction in 2008 (commit 7a5ca86). However, the implementation is bogus (actual MBR logical partitions form a sort of linked list, with one partition per extended table entry, rather than four logical partitions in a single extended table), making the code unlikely to work for anything beyond -P5 on actual guest images. What's more, the code does not support GPT partitions, which are becoming more popular, and maintaining device subsetting in both NBD and the raw device is unnecessary duplication of effort (even if it is not too difficult). Note that obtaining the offsets of a partition (MBR or GPT) can be learned by using 'qemu-nbd -c /dev/nbd0 file.qcow2 && sfdisk --dump /dev/nbd0', but by the time you've done that, you might as well just mount /dev/nbd0p1 that the kernel creates for you instead of bothering with qemu exporting a subset. Or, keeping to just user-space code, use nbdkit's partition filter, which has already known both GPT and primary MBR partitions for a while, and was just recently enhanced to support arbitrary logical MBR parititions. Start the clock on the deprecation cycle, with examples of how to write device subsetting without using -P. Signed-off-by: Eric Blake Reviewed-by: Richard W.M. Jones Reviewed-by: Stefano Garzarella --- v2: actual nbdkit example [Rich], improved doc wording --- qemu-deprecated.texi | 33 +++++++++++++++++++++++++++++++++ qemu-nbd.texi | 6 ++++-- qemu-nbd.c | 2 ++ 3 files changed, 39 insertions(+), 2 deletions(-) diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi index 219206a836f..d35e78c81ff 100644 --- a/qemu-deprecated.texi +++ b/qemu-deprecated.texi @@ -175,3 +175,36 @@ The above, converted to the current supported format: @subsubsection "irq": "" (since 3.0.0) The ``irq'' property is obsoleted. + +@section Related binaries + +@subsection qemu-nbd --partition (since 4.0.0) + +The ``qemu-nbd --partition $digit'' code (also spelled @option{-P}) +can only handle MBR partitions, and has never correctly handled +logical partitions beyond partition 5. If you know the offset and +length of the partition (perhaps by using @code{sfdisk} within the +guest), you can achieve the effect of exporting just that subset of +the disk by use of the @option{--image-opts} option with a raw +blockdev using the @code{offset} and @code{size} parameters layered on +top of any other existing blockdev. For example, if partition 1 is +100MiB long starting at 1MiB, the old command: + +@code{qemu-nbd -t -P 1 -f qcow2 file.qcow2} + +can be rewritten as: + +@code{qemu-nbd -t --image-opts driver=raw,offset=1M,size=100M,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file.qcow2} + +Alternatively, the @code{nbdkit} project provides a more powerful +partition filter on top of its nbd plugin, which can be used to select +an arbitrary MBR or GPT partition on top of any other full-image NBD +export. Using this to rewrite the above example results in: + +@code{qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &} +@code{nbdkit -f --filter=partition nbd socket=/tmp/sock partition=1} + +Note that if you are exposing the export via /dev/nbd0, it is easier +to just export the entire image and then mount only /dev/nbd0p1 than +it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a +subset of the image. diff --git a/qemu-nbd.texi b/qemu-nbd.texi index 386bece4680..d0c51828149 100644 --- a/qemu-nbd.texi +++ b/qemu-nbd.texi @@ -56,8 +56,10 @@ auto-detecting. @item -r, --read-only Export the disk as read-only. @item -P, --partition=@var{num} -Only expose MBR partition @var{num}. Understands physical partitions -1-4 and logical partitions 5-8. +Deprecated: Only expose MBR partition @var{num}. Understands physical +partitions 1-4 and logical partition 5. New code should instead use +@option{--image-opts} with the raw driver wrapping a subset of the +original image. @item -B, --bitmap=@var{name} If @var{filename} has a qcow2 persistent bitmap @var{name}, expose that bitmap via the ``qemu:dirty-bitmap:@var{name}'' context diff --git a/qemu-nbd.c b/qemu-nbd.c index 1f7b2a03f5d..00c07fd27ea 100644 --- a/qemu-nbd.c +++ b/qemu-nbd.c @@ -787,6 +787,8 @@ int main(int argc, char **argv) flags &= ~BDRV_O_RDWR; break; case 'P': + warn_report("The '-P' option is deprecated; use --image-opts with " + "a raw device wrapper for subset exports instead"); if (qemu_strtoi(optarg, NULL, 0, &partition) < 0 || partition < 1 || partition > 8) { error_report("Invalid partition '%s'", optarg);