Re: 32-bit accesses to mtime/mtimecmp under RV64


David Kruckemyer
 



On Mon, Apr 20, 2020 at 2:38 PM Andrew Waterman <andrew@...> wrote:


On Mon, Apr 20, 2020 at 11:32 AM David Kruckemyer <dkruckemyer@...> wrote:


On Fri, Apr 17, 2020 at 7:31 PM Andrew Waterman <andrew@...> wrote:


On Fri, Apr 17, 2020 at 7:00 PM Greg Favor <gfavor@...> wrote:
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)?

The spec strongly implies by omission that 64-bit accesses are atomic for RV64, in that it gives an unusually detailed RV32-specific code example to cope with non-atomicity, but mentions nothing of the sort for RV64.  I will add the additional sentence that makes this implication explicit.


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.)

In general, this depends on the peripheral and the platform.  We aren't trying to preclude interfacing with legacy devices and buses, so of course some 64-bit accesses to some devices will either become non-atomic or signal some sort of error.  But it's really quite useful to be able to assume that 64-bit accesses are atomic when interfacing with more modern peripherals that use 64-bit addresses, so we definitely do not want to preclude that, either.

Asking this slightly differently (I think) to clarify....

With respect to mtime/mtimecmp, does an RV64 processor place constraints on the platform, or can the platform place constraints on the RV64 processor? If the former, the implication is that the platform must provide a way for the RV64 processors to access the registers atomically with a 64b load or store. If the latter, the implication is that the platform can require the RV64 processor to access the registers non-atomically with 32b loads or stores, a la RV32.

The second half of my answer was addressing the more general matter. For mtime and mtimecmp specifically, the spec is now clear: 


So the only constraint is that when a 64b naturally-aligned access is made to mtime/mtimecmp, the access must be completed atomically if the platform allows 64b naturally-aligned accesses to those registers? A platform is still allowed to signal an error on such accesses and to force an RV64 processor to access those registers with 32b loads and stores, right?

Cheers,
David

 

Cheers,
David

 


Greg

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