On 26 January 2015 at 19:34, Greg Bellows <greg.bell...@linaro.org> wrote: > On Mon, Jan 26, 2015 at 8:56 AM, Peter Maydell <peter.mayd...@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
