From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45575) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aNf27-0005eT-7I for qemu-devel@nongnu.org; Mon, 25 Jan 2016 06:10:07 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aNf24-0002R7-1F for qemu-devel@nongnu.org; Mon, 25 Jan 2016 06:10:07 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39550) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aNf23-0002Q6-MR for qemu-devel@nongnu.org; Mon, 25 Jan 2016 06:10:03 -0500 Date: Mon, 25 Jan 2016 12:09:59 +0100 From: Kevin Wolf Message-ID: <20160125110959.GB5154@noname.redhat.com> References: <1452517517-3953-1-git-send-email-vsementsov@virtuozzo.com> <56981C60.9020005@redhat.com> <56982EA9.6030602@redhat.com> <569A4E71.5010004@virtuozzo.com> <569D18CE.5070104@redhat.com> <569D5648.6030605@redhat.com> <569DFA95.5020409@virtuozzo.com> <20160119172923.GG4579@noname.redhat.com> <56A5F5BD.40501@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56A5F5BD.40501@virtuozzo.com> Subject: Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vladimir Sementsov-Ogievskiy Cc: famz@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, den@openvz.org, John Snow Am 25.01.2016 um 11:15 hat Vladimir Sementsov-Ogievskiy geschrieben: > On 19.01.2016 20:29, Kevin Wolf wrote: > >Am 19.01.2016 um 09:57 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>On 19.01.2016 00:16, Eric Blake wrote: > >>>preserving semantics of those extra_data bytes). We > >>>have enough room for future extension, and that's good e > >>Ok, so, what should go to the spec? Current wording is ok? Just > >>delete "Type-specific": > >> > >>+ > >>+ 20 - 23: extra_data_size > >>+ Size of type-specific extra data. > >>+ > >>+ For now, as no extra data is defined, extra_data_size is > >>+ reserved and must be zero. > >>+ > >>+ variable: Extra data for the bitmap. > >Please be explicit that if extra_data_size is non-zero, the bitmap must > >not be used (i.e. specify the incompatible-feature-bit-like behaviour). > > It is not enough. If there are some unknown extra data, then just > ignoring this bitmap may lead to its inconsistency. So, if it is > non-zero, the whole image should not be written. (real > incompatible-feature-bit behavior). Don't we generally ignore all bitmaps until a user actively tries to make use of it? Of course, with the 'auto' flag set, just ignoring the bitmap isn't possible, but I think in all other cases it should be. If we ever add another type of bitmaps that doesn't have the 'auto' flag set, but is still automatically used, so that the image as a whole must become read-only without the bitmap, we can still add a normal incompatible feature flag. But I think it's more likely that we add extra_data that doesn't prevent use of the image, so we should have a way to express that. Kevin