On Wed, Jan 25, 2023 at 12:30:13PM +0000, Lad, Prabhakar wrote: > Renesas RZ/Five RISC-V SoC has Instruction local memory and Data local > memory (ILM & DLM) mapped between region 0x30000 - 0x4FFFF. When a > virtual address falls within this range, the MMU doesn't trigger a > page fault; it assumes the virtual address is a physical address which > can cause undesired behaviours. Wow. I've never come across such broken behaviour before. > To avoid this the ILM/DLM memory regions are now added to the root > domain region of the PMPU with permissions set to 0x0 for S/U modes so > that any access to these regions gets blocked and for M-mode we grant > full access (R/W/X). This prevents any users from accessing these > regions by triggering an unhandled signal 11 in S/U modes. I have no idea what any of this means. > This works as expected but for applications say for example when doing > mmap to this region would still succeed and later down the path when > doing a read/write to this location would cause unhandled signal 11. > To handle this case gracefully we might want mmap() itself to fail if > the addr/offset falls in this local memory region. No, that's not what you want. You want mmap to avoid allocating address space in that virtual address range. I don't know if we have a good way to do that at the moment; like I said I've never seen such broken hardware before. I'd say the right way to solve this is to add a new special kind of VMA to the address space that covers this range. We'd want to make sure it doesn't appear in /proc/*/maps and also that it can't be overridden with MAP_FIXED.
