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Re: [Xen-devel] [PATCH] [Xend] Move some backend configuration

To: John Levon <levon@xxxxxxxxxxxxxxxxx>, Pascal Bouchareine <pascal@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] [Xend] Move some backend configuration
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 30 Sep 2008 16:02:32 +0100
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Thread-topic: [Xen-devel] [PATCH] [Xend] Move some backend configuration
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On 30/9/08 15:48, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

> On 30/9/08 15:43, "John Levon" <levon@xxxxxxxxxxxxxxxxx> wrote:
>> On Tue, Sep 30, 2008 at 04:24:43PM +0200, Pascal Bouchareine wrote:
>>> This patch moves some dom0 variables and backend device
>>> configuration from frontend directories to
>>> /local/domain/<backdomid>/backend or /vm.
>> What is the point of this? These paths, however wrong they might be, are
>> API, surely.
> Which guaranteed API would that be? These paths are private to the toolstack
> implementation. Perhaps the only exception is the
> xenconsoled-to-console-client xenstore path, but that is the one that most
> urgently needs to change, since we can't trust domUs not to mess with the
> tty path, for example.

Actually I'm wrong: avoiding finding backend paths by looking in
frontend-writable xenstore nodes is even more important. But that bit of the
patch should be uncontentious: it's just xend adding extra private state in
xenstore for its own private use.

The 'helper' functions added to libxenstore perhaps make this look more like
an interface change than it actually is. Noone is forced to use those extra
utility functions, and they're only going to make sense when the /vm_path/
stuff has been set up by xend. Arguably those functions should be
implemented in the xend python wrapper for xenstore, and perhaps duplicated
as necessary in the console client (or arguably the client could safely
assume the console daemon is in dom0 and hardcode the backend path prefix!).

The console client-to-daemon interface change is unfortunate, but I can't
think of an easy alternative fix.

 -- Keir

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