Re: [Qemu-devel] [PATCH v7 05/32] target-arm: make arm_current_el() return EL3

[Peter Maydell]

On 21 October 2014 at 17:55, Greg Bellows <address@hidden> wrote:
> From: Fabian Aggeler <address@hidden>
>
> Make arm_current_el() return EL3 for secure PL1 and monitor mode.
> Increase MMU modes since mmu_index is directly inferred from arm_
> current_el(). Change assertion in arm_el_is_aa64() to allow EL3.

> -#define NB_MMU_MODES 2
> +#define NB_MMU_MODES 4

So this turns out not to quite be what we want.
A QEMU MMU mode index basically defines a (vaddr -> paddr,permissions)
mapping. This is similar to the ARM ARM concept of a "translation
regime", with the differences that:
 * the ARM ARM translation regimes may have split permissions,
   for user and privileged code, so we need two mmu_idx values
   for a translation regime that applies to both EL0 and EL1
 * stage 1 and stage 2 translations for a VA->IPA->PA lookup
   for an EL1/EL0 hypervisor guest are two different translation
   regimes, but for QEMU we can just cache the whole VA->PA
   and use a single mmu_idx. [We only need to separately do
   VA->IPA and IPA->VA for the "do this address translation"
   system instructions, which don't need to touch the TLB;
   a combined stage1+stage2 TLB is permitted by the architecture.]

The translation regimes are:

If EL3 is 64-bit:
 * Secure EL3
 * Secure EL1 & EL0
 * NonSecure EL2
 * NonSecure EL1 & 0 stage 1
 * NonSecure EL1 & 0 stage 2
If EL3 is 32-bit:
 * Secure PL0 & PL1
 * NonSecure PL2
 * NonSecure PL1 & 0 stage 1
 * NonSecure PL1 & 0 stage 2
(reminder: for 32 bit EL3, Secure PL1 is *EL3*, not EL1.)

which we can give the following mmu indexes:

64 bit EL3:
 0 : NS EL0 stage 1+2
 1 : NS EL1 stage 1+2
 2 : NS EL2
 3 : S EL3
 4 : S EL0
 5 : S EL1

32 bit EL3:
 0 : NS EL0 (aka NS PL0) stage 1+2
 1 : NS EL1 (aka NS PL1) stage 1+2
 2 : NS EL2 (aka NS PL2)
 3 : S EL3 (aka S PL1)
 4 : S EL0 (aka S PL0)

Notice how they end up being the same, except that with a
64 bit EL3 we need an extra mmu index that 32 bit doesn't have.
They aren't simply "what is our current EL?", though as you
can see I've put them in an order that comes close.

So the right answer for NB_MMU_MODES is 6 :-)

-- PMM