[U-Boot] [PATCH 1/3] ARM: HYP/non-sec: save and restore stack

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> From: Alexander Graf <agraf@suse.de>
> Date: Tue, 12 Jun 2018 22:32:38 +0200
> 
> On 12.06.18 22:17, Mark Kettenis wrote:
> >> From: Alexander Graf <agraf@suse.de>
> >> Date: Tue, 12 Jun 2018 20:46:02 +0200
> >>
> >> On 12.06.18 19:27, Mark Kettenis wrote:
> >>> The current code that switches into HYP mode doesn't bother to set
> >>> up a stack for HYP mode.  This doesn't work for EFI applications
> >>> as they expect a usable stack.  Fix this by saving the stack
> >>> pointer before switching and use it to set SP_hyp from monitor.
> >>> This restores the stack pointer when we drop into HYP mode.
> >>>
> >>> Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
> >>
> >> Can we be sure that the stack in MON is usable from HYP?
> > 
> > I think so.  It is the stack that U-Boot sets up for itself in normal
> > memory.  As far as I can tell arm64 re-uses this stack when dropping
> > down into EL2 as well.
> 
> Well, the question is whether it's secure or non-secure memory. Usually
> the DRAM controller can be configured to have a window of RAM only
> available to secure and I'd certainly hope that at least the U-Boot
> parts that are preserved in EL3 live in such a secured area :)

The U-Boot PSCI implementation ends up in a special memory region and
uses a separate stack in that same region.  Whether that memory region
is marked as secure in hardware depends on board-specific code.  On
the i.MX7D board I'm playing with it ends up in on on-chip RAM but I'm
not sure the current U-Boot code actually marks that region as secure.
In principle the PSCI code is all that is preserved for MON/EL3.  My
diffs really don't change how that works.  The code is already there
and it already works (I assume) when booting Linux kernels in
non-secure mode the traditional (non-EFI) way.