Re: [Qemu-devel] [PATCH 05/11] target-arm: Use correct mmu_idx for unprivileged loads and stores

[Peter Maydell <peter.maydell@linaro.org>]

On 26 January 2015 at 19:34, Greg Bellows <greg.bellows@linaro.org> wrote:
> On Mon, Jan 26, 2015 at 8:56 AM, Peter Maydell <peter.maydell@linaro.org>
> wrote:
>> Because that's what the ARM ARM specifies. Compare C3.2.5 (A64 LDT &c)
>> with F7.1.95 (A32/T32 LDRT).
>
>
> I had been comparing the wording of ARMv8 - F1.6.3 and ARMv7 - A4.6.3.
> After comparing the LDRT instructions between A64 (C6.6.97) and A32
> (F7.1.95), I am still missing the distinction that warrants the following
> different behavior:
>
> - EL2 is unpredictable in both A64 and A32, but in one case we treat it as
> such and the other we demote it to NS/EL0 to allow it.

No, it's not unpredictable in A64. It behaves as if a normal
(EL2) access [C3.2.5 "if the PE is executing in any other Exception
level, then a normal memory access for that level is performed"].
It is only unpredictable at EL2 in A32/T32 [F7.1.95 "UNPREDICTABLE
in Hyp mode"; in the v7 ARM ARM, A4.6.3 "UNPREDICTABLE if executed
at PL2"]. You'll see that the pseudocode for A32/T32 LDRT has
an UNPREDICTABLE check for PL2, but the pseudocode for A64
LDTR does not have any equivalent check.

> - EL3 is demoted to S/EL0 in one case but remains EL3 in the
> other.

Remains EL3 for AArch64 (by the same C3.2.5 requirement quoted above);
must act as if EL0 for AArch32 (F7.1.95 "as if the PE were running
in User mode").

This is because an EL3 A32/T32 insn is PL1, and AArch32 accesses
from PL1 must behave as if from PL0 (otherwise pre-v8 software
would break). An EL3 A64 insn, on the other hand, is definitely
not EL1 and there's no back-compatibility behaviour required.

Both these differences are required by the spec.

-- PMM