[U-Boot] Question re HYP mode and IRQ/FIQ stack setting

[Christoffer Dall]

On 12 November 2013 13:29, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:
> Hi Christoffer,
>
> On Tue, 12 Nov 2013 09:09:23 -0800, Christoffer Dall
> <christoffer.dall at linaro.org> wrote:
>
>> I suspect that if you are in Hyp mode, you should not worry about
>> FIQ/IRQ mode, but just make sure to configure Hyp mode properly to
>> handle interrupts.  (it's a separate entry in the exception vector and
>> you probably need to look at the HSR register whn you've taken an
>> interrupt).  So, as Andre suggests below, it depends on your use case.
>
> Ok, so les me try to sum that up from my perspective (IRQ and FIQ
> stacks in U-Boot):
>
> Some ARM U-Boot targets run in SVC32 mode, and some in HYP mode.
>
> For targets which run in SVC32 mode: aborts execute in abort mode, IRQs
> in IRQ mode, FIQs in FIQ mode, etc., each mode having its own stack.

correct

>
> For targets which run in HYP mode: aborts, FIQ and IRQs all run in HYP
> mode, using always the same stack.

correct.

>
> In both types of targers: prefetch and data aborts, FIQs and IRQs
> execute through the usual vectors; in SVC32 mode, because there's no
> other way; in HYP mode, because exceptions occurring while in HYP
> mode use the normal vector, not the HYP vector.
>
> Correct?
>

no, this sounds fishy. Look at the ARM ARMv7 (DDI 0406C.b) page
B1-1166.  Table B1-3 will tell you what you need to know.

Hyp mode has its own vectors, pointed to by the HVBAR control register.

I strongly suggest you familiarize yourself with these parts of the
ARM ARM before writing code to that effect.

> If so, then U-Boot stack requirements for SVC32 and HYP mode are
> as follows: in SVC32, we need separate IRQ and FIQ stacks, and the main
> (SVC32) stack does not have to accommodate for interrupt handler
> context storage. In HYP mode, the only stack is the HYP one and
> exception handlers will use it too, so it has to accommodate for
> their context storage.
>
That is correct, your Hyp mode stack should always be valid (have a
page or so, like the kernel) and then you can always push things on
there, even when taking an exception.

-Christoffer