debian stretch dom0 + xen 4.9 fails to boot

[Paul Durrant <Paul.Durrant@citrix.com>]

I've been having fun setting up a new test rig...

I have a skull canyon NUC and I put debian stretch (rc4) on it (so that's a 4.9 kernel) and then tried building and installing the latest Xen staging-4.9 code. The system failed to boot... basically it got stuck before even managing to get sufficiently into Xen to spit out anything on the console. Xen 4.8 OTOH booted just fine so I started bisecting and after 14 iterations I got down to the following commit is being the problem:

commit c0655e492e6b33e26ec9cd33f59725d0db89cdd0
Author: Juergen Gross <jgross@suse.com>
Date:   Fri Mar 24 14:18:54 2017 +0100

    x86: split boot trampoline into permanent and temporary part

    The hypervisor needs a trampoline in low memory for early boot and
    later for bringing up cpus and during wakeup from suspend. Today this
    trampoline is kept completely even if most of it isn't needed later.

    Split the trampoline into a permanent part and a temporary part needed
    at early boot only. Introduce a new entry at the boundary.

    Reduce the stack for wakeup code in order for the permanent
    trampoline to fit in a single page. 4k of stack seems excessive, about
    3k should be more than enough.

    Add an ASSERT() to the linker script to ensure the wakeup stack is
    always at least 3k.

    Signed-off-by: Juergen Gross <jgross@suse.com>
    Reviewed-by: Jan Beulich <jbeulich@suse.com>

To verify this I checked out master, reverted that commit, and tried again. The NUC still booted fine.

  Paul

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