[U-Boot] [PATCH v5 3/8] ARM: add assembly routine to switch to non-secure state

[Mj Embd]

Hello Christoffer,

I agree with both of you points.

What I found different in 2 approaches is that in your approach
S->Monitor->NS->Hyp using svc and hvc

While the other approach is setting the M bits directly in cpsr

Xen uses the following

cpsid aif, #0x16 /* Enter Monitor Mode*/
....
...
mrs   r0, cpsr               /* Copy the CPSR */
add   r0, r0, #0x4           /* 0x16 (Monitor) -> 0x1a (Hyp) */
msr   spsr_cxsf, r0          /* into the SPSR */
movs  pc, r3                 /* Exception-return into Hyp mode */

The second one is a bit simpler as it does not involve fault handlers.

I was just suggesting that the best approach to be used ...

Best Regards,
mj

On 9/20/13, Christoffer Dall <christoffer.dall at linaro.org> wrote:
> On Fri, Sep 20, 2013 at 01:27:48AM +0530, Mj Embd wrote:
>> two quick points
>> (a) xen already has a mode_switch code, so AFAIK xen might not use it
>> (as suggested by comment in another patch in this patch set)
>
> For KVM the boot procedure for Hyp mode is quite clearly defined: the
> kernel must be booted in Hyp mode.
>
> I was under the impression that Xen wanted to use the same paradigm and
> rely on bootloaders to switch to Hyp mode and thereby get rid of the
> code in Xen.
>
>> (b) There are 2 methods of switching from Secure to Hyp mode
>> one you have proposed another implemented in xen. I was suggesting
>> take the best approach
>>
>
> Can you please be more specific?  Not everyone here knows the Xen
> low-level mode switch details by heart.  As far as I know, there is only
> one architecturally defined proper mode to switch from secure mode to
> non-secure mode, and the state that needs to be configured for Hyp-mode
> and NS-mode is well defined.  Obviously two implementation can do things
> differently (different order, different programminge environment, etc.)
> but that doesn't mean one is better than the other.
>
> I think it would be more productive if you can simply look at this code
> and if you think some things are done more efficiently in Xen, please
> comment on that, which would be very helpful.  I'm afraid there's no
> magic way to apply a block of Xen code into U-Boot wihtout manual
> adjustment anyway, or the other way around for that matter.
>
> -Christoffer
>


-- 
-mj