From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:42503)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <armbru@redhat.com>) id 1aqNU1-0006ZV-9C
	for qemu-devel@nongnu.org; Wed, 13 Apr 2016 12:17:38 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <armbru@redhat.com>) id 1aqNTv-0006Az-LG
	for qemu-devel@nongnu.org; Wed, 13 Apr 2016 12:17:37 -0400
Received: from mx1.redhat.com ([209.132.183.28]:55090)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <armbru@redhat.com>) id 1aqNTv-0006Av-GW
	for qemu-devel@nongnu.org; Wed, 13 Apr 2016 12:17:31 -0400
From: Markus Armbruster <armbru@redhat.com>
References: <1460102035-15972-1-git-send-email-mst@redhat.com>
	<87twjcvz7o.fsf@dusky.pond.sub.org>
	<20160409200141-mutt-send-email-mst@redhat.com>
	<8760voo18x.fsf@dusky.pond.sub.org>
	<20160411142509-mutt-send-email-mst@redhat.com>
	<8760vlq3yg.fsf@dusky.pond.sub.org>
	<20160413135342-mutt-send-email-mst@redhat.com>
Date: Wed, 13 Apr 2016 18:17:29 +0200
In-Reply-To: <20160413135342-mutt-send-email-mst@redhat.com> (Michael
	S. Tsirkin's message of "Wed, 13 Apr 2016 14:00:39 +0300")
Message-ID: <87h9f5iiue.fsf@dusky.pond.sub.org>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [Qemu-devel] [PATCH v3] fw_cfg: RFQDN rules, documentation
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel/>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "Gabriel L . Somlo" <somlo@cmu.edu>, Laszlo Ersek <lersek@redhat.com>, qemu-devel@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Wed, Apr 13, 2016 at 10:59:35AM +0200, Markus Armbruster wrote:
>> I have a hard time coming up with realistic unclean breakage.
>
> The issue is that Linux is now exposing fw cfg to userspace.
> So it's use is about to expand significantly.
>
> This is what I am trying to prevent:
> - in 2016, users build a guest using a path XXX outside opt.
>   there's a warning on host, but it is not noticed.

Amend:

    The guest treats path XXX as optional.

> - in 2020, qemu starts using path XXX for internal purposes.
> - using guest from 2016 now breaks uncleanly on this new qemu

Amend:

    when we're not specifying the optional path XXX with -fw_cfg.

>   since guest thinks it's talking to the external tool.

Okay, that's a much more plausible scenario.  The question remains
whether preventing it justifies the compat break and the additional
interface complexity.