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From: Markus Armbruster <armbru@redhat.com>
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Date: Thu, 14 Apr 2016 09:36:28 +0200
In-Reply-To: <20160413205343-mutt-send-email-mst@redhat.com> (Michael
	S. Tsirkin's message of "Wed, 13 Apr 2016 20:53:58 +0300")
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Subject: Re: [Qemu-devel] [PATCH v3] fw_cfg: RFQDN rules, documentation
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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 06:17:29PM +0200, Markus Armbruster wrote:
>> "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.
>
> there is no break as long as people follow the rules.

-fw_cfg exists since 2.4.  You can't slap rules onto it in 2.6, and
immediately claim compatibility matters only for usage following these
rules.