Prevent speculative access using PRIVDEFENA
After facing some weird stability issues on the STM32F7 we have setup the MPU according to the recommendations presented in "MPU usage in STM32 with ARM CortexM7":
As far as I understand, the key point is to prevent speculative execution fetch from ExternalRAM and ExternalDevice memory regions. This is done by setup a 4GB wide low priority region and then enabling only some sub-regions (0x87) to have a range from 0x6000 0000 to 0xDFFF FFFF:
Since MPU_PRIVILEDGED_DEFAULT is enabled in this example, for all disabled subsections of the MPU_REGION_NUMBER0 the default memorymap will be active (privileged access only).
My question is: Could the speculative access also be prevented by disabling the privileged default memory map (PRIVDEFENA = 0). This would disallow all access (R/W) to the whole memory range at first. Memory areas used by the application would then have to be added defining separate MPU regions.