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Re: [Xen-devel] [PATCH] Paravirt framebuffer frontend kernel support [1/

To: Markus Armbruster <armbru@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Paravirt framebuffer frontend kernel support [1/5]
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 21 Sep 2006 20:29:26 +0100
Cc: Jeremy Katz <katzj@xxxxxxxxxx>, aliguori <aliguori@xxxxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, sos22@xxxxxxxxxxxxx
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On 21/9/06 7:41 pm, "Markus Armbruster" <armbru@xxxxxxxxxx> wrote:

> Is there anything we can do now to help with maintaining backward
> compatibility later?
> Evolving interfaces are a fact of life.  What about versioning?
> Frontend puts its interface version in xenstore, bump it when we
> change stuff (which should happen very rarely, of course), backend
> queries the version and does the right thing.

We don't need to think too hard about this right now. The way we do this for
netfront/netback is to add feature nodes to xenstore. Absence of a feature
node (e.g., because backend is too old to support the feature) means the
feature is unavailable to the frontend and it will fall back to old methods.
In some cases feature negotiation is a handshake (backend writes a feature
node to indicate it supports a feature; frontend writes a request node to
indicate it wants to use a supported feature).

 -- Keir

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