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Re: [Xen-devel] [RFC] ARM VM System Sepcification

On Thu, 6 Mar 2014 12:04:50 +0000, Robie Basak <robie.basak@xxxxxxxxxxxxx> 
> On Thu, Mar 06, 2014 at 12:44:57PM +0100, Laszlo Ersek wrote:
> > If I understand correctly, the question is this:
> > 
> >   Given a hypervisor that doesn't support non-volatile UEFI variables
> >   (including BootOrder and Boot####), is it possible to automatically
> >   boot a carefully prepared VM image, made available as a fixed media
> >   device?
> > 
> > The answer is "yes". See
> Right, but I think there is a subsequent problem.
> >   It is expected that this default boot will load an operating system or
> >   a maintenance utility. If this is an operating system setup program it
> >   is then responsible for setting the requisite environment variables
> >   for subsequent boots. The platform firmware may also decide to recover
>         ^^^^^^^^^^^^^^^^
> >   or set to a known set of boot options.
> It seems to me that the guest OS is permitted to assume that persistent
> boot variables will work after first boot, for subsequent boots.
> So, for example, the guest OS might, on bootloader or kernel upgrade,
> completely replace the boot mechanism, dropping the removable path and
> replacing it with a fixed disk arrangement, setting boot variables
> appropriately, and assume that it can reboot and everything will
> continue to work.
> But if the host does not support non-volatile variables, then this will
> break.


> This is why I'm suggesting that the specification mandate that the guest
> OS shipped in a "portable disk image" as defined by the spec must not
> make this assumption.

Also correct... the installer must be aware of this constraint which is
why it is part of the draft spec.

> It's either this, or mandate that all hosts must support persistent
> variables. I have no objection to that in principle, but since we have
> no implementation currently, it seems easier to avoid this particular
> roadblock by tweaking the spec in a way that nobody seems to care about
> anyway.

Right. I guess my position is that if persistent storage is not
implemented then there are a number of install/upgrade scenarios that
won't work. Regardless, portable images must assume an empty boot list
and we can build that into the spec.


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