Re: flat mapping vs identity mapping on ARM

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

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