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Re: [Xen-devel] [PATCH, v2] add privileged/unprivileged kernel feature i

To: Jan Beulich <JBeulich@xxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH, v2] add privileged/unprivileged kernel feature indication
From: Keir Fraser <keir@xxxxxxx>
Date: Thu, 21 Jul 2011 09:55:56 +0100
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Thread-topic: [Xen-devel] [PATCH, v2] add privileged/unprivileged kernel feature indication
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On 21/07/2011 09:50, "Keir Fraser" <keir@xxxxxxx> wrote:

> On 21/07/2011 09:16, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>>> You say it is a Linux notion that dom0 implies domU but I am not aware
>>> of any PV OS which supports dom0 that doesn't also support domU, do you
>>> have specific examples of OSes which are dom0-only?
>> No, I'm not aware of any existing ones, but I also wasn't in favor of
>> the move to imply unprivileged capabilities when Linux is configured
>> as privileged guest (iirc this wasn't the case from the very beginning).
>> And again, imo an interface like the hypervisor's shouldn't dictate any
>> kind of policy on the guest OSes.
> My own issue with the unprivileged flag is that I'm not clear what it
> actually means. When would you *not* set it? I mean it looks in the Linux
> side you set it unconditionally right now. What's the point? Why not remove
> the flag and introduce it when we have good reason and can attach meaningful
> semantics to it?

A further killing blow: the hypervisor patch defined unprivileged as !dom0.
Well, there are many different capabilities and devices that a domU may be
granted. You might be passing through a VGA adaptor and SRIOV NIC and run
out of ramdisk for example, in which case the domU might quite validly have
no PV frontend devices.

Another thing, given that privileged is quite a broad term, I wonder whether
the 'privileged' feature should be called something else? Like
'dom0_interface'? It would be a more precise definition maybe? Passing
through devices to a domU could be termed a privilege after all, for

 -- Keir

> There we are, we're two against one now ;-)
>  -- Keir

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