i find some BUG when use bsp with mmu
1. all device memory must set XN bit. device memory Instruction fetches not allowed (always treated as XN=1).(see armv7 arm-arm ***B5.2.1 Access permissions***) On our board this case fetches Instruction in device memory. This bug is quite troublesome to find!
2. link.cmd MMU memory must align by page size(1M or 4K ). if not , may case Page property override!!
3. may support page 1m an 4K mix. i have change a version and it work well. this will save memory.
4. can use MMU to do stack check. stack will map twice ，
(1)one-one mapping
(2) mapping with hole. like
{|-----------other virtual space ------------| stack (map phy memory) | --- page size hole (only has virtual memory , not map phy memory )---| stack2| --- page size hole (only has virtual memory , not map phy memory )---| -----------other virtual space ------------|}
5. memory with mpu, can use a region protect stack last 32 byte , when task switch , switch stack to other task's stack.
4 and 5 , We're implementing the code for test.
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