ARM processor mode, kernel startup, Hyp / secure state

[Will Deacon]

Dave,

On Tue, Aug 23, 2011 at 05:50:19PM +0100, Dave Martin wrote:
> On Tue, Aug 23, 2011 at 3:52 PM, Ian Jackson <Ian.Jackson at eu.citrix.com> wrote:
> > At first I thought that the best thing to do would be to boot the
> > kernel in any suitable mode, and have the kernel automatically detect
> > the starting mode.  I started writing code in linux's head.S to do
> > this.  However, detecting whether we are in secure state is very
> > difficult: it involves deliberately risking an undefined instruction
> > trap.  The code for this was getting rather long and involved.
> 
> There may be a safe way to do this check -- for example, on ARM1176
> and Cortex-A8 there is a CP14 debug status/control register that you can
> read which includes a flag indicating which world you're in.  This isn't part
> of the architecture though and may be different/not possible on some
> CPUs.

Please don't do this! Accessing the debug registers via the CP14 registers
is like playing russian roulette with a machine gun, especially when you
have various hypervisor registers and hardware lock registers to contend
with. For 3.2, I will be guarding all of the hw_breakpoint init debug
probing with an undef_hook because I'm sick of blowing my head off when
systems are configured to keep debug out.

> All in all, it's better to engineer things so that the check doesn't need to
> be done at all

Agreed. I think it's better to assume that you can't detect whether you're
running in secure state or not.

Will