32-bit accesses to mtime/mtimecmp under RV64

Greg Favor

The mtime and mtimecmp registers are defined as 64-bit memory-mapped registers.  The priv spec says that - in RV32 - mtimecmp can be written as a pair of 32-bit registers.  Since this was made specific to RV32, is there an intended implication in the spec that in RV64 the system must support atomic 64-bit accesses to these registers?  Or is it allowable for only non-atomic 64-bit accesses to be supported (i.e. a 64-bit access by a CPU is performed as two 32-bit accesses out in the SoC where mtime/mtimecmp are located)?

Put differently, must RV64 software not assume that a 64-bit load/store will atomically read/write the register.  (Note: ARMv8 explicitly says software must not make such an atomicity assumption for accesses to memory-mapped 64-bit registers.)


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