[U-Boot] [PATCH] armv8: Remove the codes about switching to EL1 before jumping to kernel

[Alexander Graf]

On 08/29/2016 11:29 AM, Huan Wang wrote:
>> On 18.07.16 05:24, Huan Wang wrote:
>>> Hi, Alex,
>>>
>>>
>>>
>>>                As there is strong objection to remove the codes about
>>> switching to EL1, I think we have to remain it, do you agree?
>> I agree, yes.
>>
>>>                  If it is remained, I think your suggestion about
>>> **always** jumping to ep for both switching to AArch64 and AArch32
>>> modes will make the code hard to realize and very complicated. So I
>>> prefer to  keep the process in v4 patches. What is your opinion?
>> I think we should still convert it to a function call based approach.
>> You can either just convert the current flow to functions:
>>
>> static void enter_in_el1(...)
>> {
>>      call_in_el1(payload_pc, payload_bits, ...); }
>>
>> #ifdef ENTER_PAYLOAD_IN_EL1
>> call_in_el2(enter_in_el1, 64bit, ...);
>> #else
>> call_in_el2(payload_pc, payload_bits, ...); #endif
>>
>> Or you could add a check in the EL1 caller if you are in EL3 that you
>> want to go to EL2 first:
>>
>> long call_in_el1(...)
>> {
>>    if (current_el() == 3)
>>      return call_in_el2(call_in_el1, ...);
>>
>>    asm_call_in_el1(...);
>> }
>>
>>
> [Alison Wang] Yes, it can work for primary core and secondary cores for LayerScape.
>
> For other ARMv8 platforms, such as
> arch/arm/mach-exynos/soc.c,
> void lowlevel_init(void)
> {
>          armv8_switch_to_el2();
>          armv8_switch_to_el1();
> }
> Is there any appropriate ep we can transfer for these functions?

First off, I'd be surprised if the sequence above even works at all, as 
you also need to set up your page tables for el2/el1 if you want to get 
into those.

IMHO the best path for this case is to remove the function :). But 
double-check with the Samsung folks first. I don't see why they can't 
use the generic one.

> Even for the common arch/arm/cpu/armv8/start.S,
> bl      armv8_switch_to_el2
> #ifdef CONFIG_ARMV8_SWITCH_TO_EL1
> bl      armv8_switch_to_el1
> #endif
> The ep is hard to define.

It's pretty simple. Just pass a pointer to the instruction after bl into 
the switch function:

         /*
          * All slaves will enter EL2 and optionally EL1.
          */
         adr    x0, lowlevel_in_el2
         bl      asm_call_in_el2
         b        panic
lowlevel_in_el2:

#ifdef CONFIG_ARMV8_SWITCH_TO_EL1
         adr    x0, lowlevel_in_el1
         bl      asm_call_in_el1
         b        panic
lowlevel_in_el1:
#endif

Because you already save the real lr in a non-volatile register, all 
this does is waste a few bytes of stack compared to the previous code.


Alex