Re: flat mapping vs identity mapping on ARM

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 22 September 2016 at 16:22, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> On 22 September 2016 at 16:19, valerij zaporogeci <vlrzprgts@gmail.com> wrote:
>> 2016-09-22 18:03 GMT+03:00, Ard Biesheuvel <ard.biesheuvel@linaro.org>:
>>> On 22 September 2016 at 15:30, valerij zaporogeci <vlrzprgts@gmail.com>
>>> wrote:
>>>> In the ARM architecture, there is such a thing - "flat mapping", where
>>>> MMU stage 1 is disabled and the mapping done is 1:1 and attributes set
>>>> to the predefined values.
>>>
>>> What do you mean by 'attributes set to the predefined values' ?
>>>
>>
>> I meant what is written in the section B3.2.1, short quote:
>> "For all other accesses, when a stage 1 MMU is disabled, the assigned
>> attributes depend on whether
>> the access is a data access or an instruction access, as follows:
>> Data access
>> The stage 1 translation assigns the Strongly-Ordered memory type"
>>
>> ... and then, more lengthy description for instruction access.
>>
>>
>
> Yes, so what this means is that all data accesses are strongly
> ordered, and instruction fetches are cacheable.
>
> So while you can enable both the data and the instruction cache with
> the MMU off, only the instruction cache is actually functional, since
> all data accesses or non-cacheable

*are* non-cacheable