On Tue, Aug 23, 2011 at 11:18:00PM +0100, Will Deacon wrote: > 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. Heh -- I didn't think it was quite that scary, but point taken. > > 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
