Re: [Qemu-devel] [PATCH for-3.0] target/arm: Correctly handle overlapping small MPU regions

[Richard Henderson <richard.henderson@linaro.org>]

On 07/17/2018 12:40 PM, Peter Maydell wrote:
> On 17 July 2018 at 18:29, Richard Henderson
> <richard.henderson@linaro.org> wrote:
>> I don't understand why this is necessary in the v8m case.
>>
>>                AP                 APL
>>     <----B1----|----L1-B2-A-------|---L2--->
>>
>>
>> Your comment posits two regions [B1,L1] and [B2,L2], that A is not within
>> [B1,L1] but is within [B2,L2] (otherwise we would not report a hit at all).
>> Further, that [B1,L1] intersects [AP,APL] but does not intersect [B2,L2]
>> (otherwise we would report a fault for overlapping regions).
>>
>> Surely this combination of ranges implies that [B2,L2] must itself set
>> IS_SUBPAGE (otherwise the first region would not overlap the page of A, or
>> would not overlap the second region).
> 
> (a) the overlap fault is only for addresses which are actually in the
> overlap (ie "you asked about address X and it hits in R1 and R2"); it
> doesn't imply that other addresses which are only in R1 fault just
> because some part of R1 overlaps with R2.

Ah, that wasn't clear from the manual.

> So for instance:
> 
>        AP                        APL
>    [B1                                L1]
>                  [B2       L2]
>              A1^     A2^

Thanks for the example,

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~