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It’s not even always architecturally possible - ARMv8 doesn’t support user-space mappings with bit 63 set, for example.
by repiret 4y ago
It’s not even always architecturally possible - ARMv8 doesn’t support user-space mappings with bit 63 set, for example.
- moonchild 4y agoI see. I was talking specifically about x86 here (and the conventions of mainstream OSes hosted on it); obviously, every platform is unique. It looks like on arm, the high bit of an address is used as a special marker to indicate which translation table should be used to look up the rest of the address. I am no arm expert, but looking at the documentation at [0], it does not seem like allowing 'userspace' access to addresses mapped in TTBR1 would be an insurmountable challenge. 0. https://developer.arm.com/documentation/ddi0406/b/System-Level-Architecture/Virtual-Memory-System-Architecture--VMSA-/CP15-registers-for-a-VMSA-implementation/CP15-c2--Translation-table-support-registers https://developer.arm.com/documentation/ddi0406/b/System-Lev...