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Date: Wed, 13 Apr 2016 20:53:58 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3] fw_cfg: RFQDN rules, documentation
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To: Markus Armbruster <armbru@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>

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.