From patchwork Fri Jun 22 15:08:40 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Stefan Hajnoczi X-Patchwork-Id: 166638 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id C560FB6FA0 for ; Sat, 23 Jun 2012 01:10:23 +1000 (EST) Received: from localhost ([::1]:40566 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Si5V7-0008O5-JF for incoming@patchwork.ozlabs.org; Fri, 22 Jun 2012 11:10:21 -0400 Received: from eggs.gnu.org ([208.118.235.92]:54314) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Si5U0-0005Zv-PC for qemu-devel@nongnu.org; Fri, 22 Jun 2012 11:09:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Si5Tq-0007tC-M2 for qemu-devel@nongnu.org; Fri, 22 Jun 2012 11:09:12 -0400 Received: from e06smtp16.uk.ibm.com ([195.75.94.112]:48339) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Si5Tq-0007sc-Dl for qemu-devel@nongnu.org; Fri, 22 Jun 2012 11:09:02 -0400 Received: from /spool/local by e06smtp16.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 22 Jun 2012 16:08:58 +0100 Received: from d06nrmr1407.portsmouth.uk.ibm.com (9.149.38.185) by e06smtp16.uk.ibm.com (192.168.101.146) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; Fri, 22 Jun 2012 16:08:55 +0100 Received: from d06av12.portsmouth.uk.ibm.com (d06av12.portsmouth.uk.ibm.com [9.149.37.247]) by d06nrmr1407.portsmouth.uk.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id q5MF8sr62445514 for ; Fri, 22 Jun 2012 16:08:54 +0100 Received: from d06av12.portsmouth.uk.ibm.com (loopback [127.0.0.1]) by d06av12.portsmouth.uk.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id q5MF8ski013867 for ; Fri, 22 Jun 2012 09:08:54 -0600 Received: from localhost (stefanha-thinkpad.manchester-maybrook.uk.ibm.com [9.174.219.145]) by d06av12.portsmouth.uk.ibm.com (8.14.4/8.13.1/NCO v10.0 AVin) with ESMTP id q5MF8svm013858; Fri, 22 Jun 2012 09:08:54 -0600 From: Stefan Hajnoczi To: Kevin Wolf Date: Fri, 22 Jun 2012 16:08:40 +0100 Message-Id: <1340377726-5896-2-git-send-email-stefanha@linux.vnet.ibm.com> X-Mailer: git-send-email 1.7.10 In-Reply-To: <1340377726-5896-1-git-send-email-stefanha@linux.vnet.ibm.com> References: <1340377726-5896-1-git-send-email-stefanha@linux.vnet.ibm.com> x-cbid: 12062215-3548-0000-0000-00000251D5C6 X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 195.75.94.112 Cc: qemu-devel@nongnu.org, Stefan Hajnoczi Subject: [Qemu-devel] [RFC 1/7] docs: add dirty bit to qcow2 specification X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org The dirty bit will make it possible to perform lazy refcount updates, where the image file is not kept consistent all the time. Upon opening a dirty image file, it is necessary to perform a consistency check and repair any incorrect refcounts. Therefore the dirty bit must be an incompatible feature bit. We don't want old programs accessing a file with stale refcounts. Signed-off-by: Stefan Hajnoczi --- docs/specs/qcow2.txt | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt index 87bf785..3a789ce 100644 --- a/docs/specs/qcow2.txt +++ b/docs/specs/qcow2.txt @@ -75,7 +75,11 @@ in the description of a field. Bitmask of incompatible features. An implementation must fail to open an image if an unknown bit is set. - Bits 0-63: Reserved (set to 0) + Bit 0: Dirty bit. If this bit is set then refcounts may be + inconsistent, make sure to scan L1/L2 tables to + repair refcounts before accessing the image. + + Bits 1-63: Reserved (set to 0) 80 - 87: compatible_features Bitmask of compatible features. An implementation can