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

On Sat, Mar 22, 2014 at 08:19:52PM -0700, Christoffer Dall wrote:
> On Sat, Mar 22, 2014 at 09:08:37AM +0100, Paolo Bonzini wrote:
> > Il 22/03/2014 03:29, Christoffer Dall ha scritto:
> > >1. Simply mandate that VM implementations support persistent variables
> > >   for their UEFI implementation - with whatever constraints that may
> > >   put on higher level tools.
> > >
> > >2. Require that OSes shipped as part of compliant VM images make no
> > >   assumption that changes to the UEFI environment will be stored.
> > >
> > >I feel that option number two will break in all sorts of cases, just
> > >like Grant stated above, and it is fundamentally not practical; if a
> > >distribution ships Linux with a UEFI stub that expects to be able to do
> > >something, distributions must modify Linux to conform to this spec.  I
> > >think imagining that this spec controls how UEFI support in Linux/Grub
> > >is done in general would be overreaching.  Additionally, Michael brought
> > >up the fact that it would be non-UEFI compliant.
> > 
> > OSes are already able to cope with loss of changes to UEFI
> > environment are stored, because losing persistent variables is what
> > happens if you copy an image to a new hard disk.
> > 
> > Asking implementations for support of persistent variables is a good
> > idea; however, independent of what is in the spec, OSes should not
> > expect that users will enable that support---most of them won't.
> > 
> OK, fair enough, mandating support for persistent variable storage may
> be overreaching but at the same time I feel it is unlikely for this spec
> to reach far enough that a generic UEFI Linux loader, for example,
> actually follows it, and therefore explicitly mandating that guest OSes
> must be completely portable in all that they do is a non-practical
> constraint.  That was the request that kicked this discussion off, and
> what I was trygint to address.
After thinking about this a bit more, I think I see what we're actually
discussing.  It's obvious that if software in a VM makes changes to UEFI
variables that are required to be persistent for that VM image to boot
again, then the VM image is no longer portable, as per the spec.

It is in fact probably a good idea to specify that out clearly, I found
it intuitively obvious, but it could be spelled out nevertheless.


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