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

[Christoffer Dall]

On 12 November 2013 03:41, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:
> (Cc:ing Andre and Christoffer as they have discussed HYP on the ML.)
>
> Hello,
>
> I am working on changing the way IRQ/FIQ stacks are set up, from
> "on-the-fly in a hurry while in the handler" to "during init, so that
> when entering the handler, the stack is already correct".
>
> Setting the stack then requires switching from the current mode (in
> most cases, SVC32, 0x13) to IRQ (0x11) or FIQ (0x12) mode, in order to
> set the right banked SP, then back into the original mode.
>
> However, in the first lines of reset in arch/arm/cpu/armv7/start.S, the
> possibility of U-Boot being started in HYP mode (0x1A) is considered
> and, if in HYP mode, no switch to SVC32 is performed.
>
> I understand that the problem here is, if we drop from HYP to SVC32,
> then we cannot go back to HYP, and we want to be able to remain in HYP.

correct (not without setting up a trap handler in Hyp mode and
trapping to Hyp mode)

>
> Does this also apply to dropping from HYP to IRQ or FIQ mode, i.e., if
> we do such a drop, are we prevented from rising back from IRQ or FIQ
> mode to HYP? I seem to remember such an issue, but I am no specialist
> in HYP, so any help is welcome.

Yes, it also applies.  Hyp is strictly more privileged (PL2) than all
the PL1 modes (SVC, SYS, IRQ, FIQ, ABT, UND) and therefore requires a
trap to go from PL1 to PL2 (basically this is how hardware protection
works - just like with syscalls from user mode to PL1).

You can use MSR and MRS instructions to access the IRQ and FIQ
registers directly from Hyp mode though.

-Christoffer