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Re: [Qemu-devel] [RFC][PATCH 0/6] "bootonceindex" property

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC][PATCH 0/6] "bootonceindex" property
Date: Wed, 22 Mar 2017 04:43:10 -0400 (EDT)

----- Original Message -----
> From: "Janne Huttunen" <address@hidden>
> To: "Paolo Bonzini" <address@hidden>, "Gerd Hoffmann" <address@hidden>
> Cc: address@hidden
> Sent: Wednesday, March 22, 2017 7:36:54 AM
> Subject: Re: [Qemu-devel] [RFC][PATCH 0/6] "bootonceindex" property
> On Tue, 2017-03-21 at 18:55 +0100, Paolo Bonzini wrote:
> > 
> > > Since real HW has this capability, there exist certain
> > > auxiliary systems that are built on it. Having similar
> > > semantics available in QEMU allows me to build a virtual
> > > machine that works with these systems without modifying
> > > them in any way.
> >
> > How does real hardware do it?  I suppose you'd do it with a firmware
> > setup menu or something like that; would it be enough to add a way to
> > modify bootindex during runtime?
> On the real hardware the "boot once" really means *once*
> i.e. it only affects the next reboot regardless of how
> the next boot is triggered (reset button, power button,
> software, etc.). After the next boot the normal boot
> order is automatically restored.

Understood---my question is how you would set up the alternate
boot order: is it something like "keep a button pressed while
turning on", or something written in NVRAM, or something else
that is completely different?


> Theoretically it should be possible to get a close
> approximation of this by changing the main boot order,
> waiting for the boot to happen and then restoring the
> original order back. This would require having some
> process that constantly monitors what QEMU is doing so
> that it can notice when the boot happens and then
> restore the order. I'm trying to avoid having such
> a process if possible, which in this case means that
> QEMU would need to restore the order by itself.

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