From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34967) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XtByG-0004JC-EZ for qemu-devel@nongnu.org; Tue, 25 Nov 2014 03:59:45 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XtByB-00051m-Uy for qemu-devel@nongnu.org; Tue, 25 Nov 2014 03:59:40 -0500 Date: Tue, 25 Nov 2014 08:59:15 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20141125085915.GA2550@work-vm> References: <1416259239-13281-1-git-send-email-dslutz@verizon.com> <87k32r8jc8.fsf@blackfin.pond.sub.org> <546E339E.4030202@terremark.com> <87fvdd6j29.fsf@blackfin.pond.sub.org> <20141121104945.GH3165@work-vm> <5473D1CE.7040102@terremark.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5473D1CE.7040102@terremark.com> Subject: Re: [Qemu-devel] [BUGFIX][PATCH for 2.2 1/1] hw/ide/core.c: Prevent SIGSEGV during migration List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Don Slutz Cc: Kevin Wolf , Stefano Stabellini , qemu-stable@nongnu.org, qemu-devel@nongnu.org, Markus Armbruster , Stefan Hajnoczi * Don Slutz (dslutz@verizon.com) wrote: > On 11/21/14 05:49, Dr. David Alan Gilbert wrote: > >* Markus Armbruster (armbru@redhat.com) wrote: > >>Don Slutz writes: > >> > >>>On 11/19/14 07:29, Markus Armbruster wrote: > >>>>Don Slutz writes: > >>>> > >>>>>The other callers to blk_set_enable_write_cache() in this file > >>>>>already check for s->blk == NULL. > >>>>> > >>>>>Signed-off-by: Don Slutz > >>>>>--- > >>>>> > >>>>>I think this is a bugfix that should be back ported to stable > >>>>>releases. > >>>>> > >>>>>I also think this should be done in xen's copy of QEMU for 4.5 with > >>>>>back port(s) to active stable releases. > >>>>> > >>>>>Note: In 2.1 and earlier the routine is > >>>>>bdrv_set_enable_write_cache(); variable is s->bs. > >>>>Got a reproducer? > >>>yes. Migrating a guest from xen 4.2 or 4.3 to xen 4.4 (or 4.5-unstable) on > >>>CentOS 6.3 with xen_emul_unplug=unnecessary and no cdrom defined. > >>> > >>> > >>>>I'm asking because I believe s->identify_set implies s->blk. > >>>>s->identify_set is initialized to zero, and gets set to non-zero exactly > >>>>on the first successful IDENTIFY DEVICE or IDENTIFY PACKET DEVICE, in > >>>>ide_identify(), ide_atapi_identify() or ide_cfata_identify(), > >>>>respectively. Only called via cmd_identify() / cmd_identify_packet() > >>>>via ide_exec_cmd(). The latter immediately fails when !s->blk: > >>>> > >>>> s = idebus_active_if(bus); > >>>> /* ignore commands to non existent slave */ > >>>> if (s != bus->ifs && !s->blk) { > >>>> return; > >>>> } > >>>I do think that you are right. I have now spent more time on why I am > >>>seeing this. > >>> > >>> > >>>>Even if I'm right, your patch is fine, because it makes this spot more > >>>>obviously correct, and consistent with the other uses of > >>>>blk_set_enable_write_cache(). The case for stable is weak, though. > >>>> > >>>I had not fully tracked down what is happening before sending the bugfix. > >>>I have now done more debugging, and have tracked it down to xen 4.4 > >>>now using "-nodefaults" with QEMU. > >>> > >>>I needed to add output to QEMU to track this down because I have long > >>>command lines... > >>> > >>>(all I get for ps -ef): > >>[...] > >>>Which is missing that option. > >>> > >>>The ide that was aborting in this case is the cdrom at hdc that is added > >>>if you do not specify "-nodefaults". > >>> > >>>Since this is a "changed" machine config, I am no longer as sure as what > >>>versions this needs to be in. > >>> > >>>If I put my QEMU hat on, it does not look like a back port is needed. > >>>However > >>>for xen it would be nice. > >>> > >>>I do not know how the QEMU community feels about migration from a config > >>>without "-nodefaults" to one with "-nodefaults" as the only difference. > >>So you have a CD-ROM on the source, but not on the destination? > > Yes, QEMU in the source has an empty CD-ROM, but not on the destination. > xen > does not know that QEMU added a CD-ROM drive in hdc by default. > > >>That can't work. I guess it broke for you in an unusual way (target > >>crashes) rather than the usual way (target rejects migration data for a > >>device it doesn't have) due to our convoluted IDE data structures. With > >>your patch applied it should break the usual way. Does it? > > Nope. It does not break at all. The migration "works". The target accepts > the hdc data. > It looks like to me that all 4 IDE state data is sent. > > > >>Management tools should use -nodefaults. But if it mixes default and > >>-nodefaults in migration, recreating the stuff it got by default but > >>doesn't get with -nodefaults is its own responsibility. > > Yes. Since xen did not ask for a CD-ROM, it did not expect to need to create > it. xen was "fixed" to use -nodefaults. > > >Well, mostly - we wouldn't expect a migration to work if the source/dest > >didn't match exactly; but QEMU shouldn't seg. > > Well, this change prevents a seg. So you are in favor of having this > backported? Yes. Dave > > > -Don Slutz > > >Dave > > > >-- > >Dr. David Alan Gilbert /dgilbert@redhat.com / Manchester, UK > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK