ARM processor mode, kernel startup, Hyp / secure state

[Ian Jackson]

Will Deacon writes ("Re: ARM processor mode, kernel startup, Hyp / secure state"):
> On Wed, Aug 24, 2011 at 05:09:01PM +0100, Ian Jackson wrote:
> > I think the trampoline needs to have one hypercall: "set HVBAR".
> > (And, implicitly, it needs to be specified that the resulting vectors
> > will run without physical-to-virtual page translation, etc.)  That is
> > sufficient to bootstrap all the rest of the way up - and that other
> > bootstrapping code would have to exist anyway.
> 
> I also thought you might want a hypercall that gives you a version number
> (i.e. a way to identify the hypervisor trampoline in case it changes)

Since the hypervisor trampoline was installed by the very same kernel
which is running, there is no need for any defined interface or
version numbers.

> and we might also want something for doing a system reset (for
> example, if a kernel that hasn't installed a hypervisor wants to
> kexec another kernel image).

No, if a kernel has installed a hypervisor and wishes to kexec then it
must start the new kernel in Hyp mode - since that is what we are
proposing to specify as the cpu mode to use when booting the kernel.

So the outgoing kernel simply switches to Hyp mode, tears down Hyp
mode's paging (and probably the rest of the paging too), and then
enters the new kernel in Hyp mode with paging off.  The new kernel
then installs its own hypervisor trampoline.

If the outgoing kernel hasn't installed a hypervisor it's even easier:
simply do the usual teardown, and then switch to Hyp mode using its
existing trampoline and then enter the new kernel.

Ian.