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Setting Write Combine memory type.



I was trying to set memory type as Write combine because I want to use 'movntdqa' instructions.

I found the set_memory_wc() function in kernel code which take the memory address and size as argument.

In that function, It convert the logical address to physical address by using __pa() function.

As far as I know __pa() function is only valid when the address is kernel logical address space.

So I don't know the set_memory_wc() function is also can be called by user process.

I have implemented the custom system call which call set_memory_wc() function and used in user process but I generate error.

The error message was " CPA: called for zero pte.". I think this error is due to I am calling that function is user address space.

Then Is there any way to set memory type to write combine in address space??

Thank you in advance.




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Black Belt

The WC memory type is typically only used for memory-mapped IO regions, not for system memory regions.

The MOVNTDQA instruction is intended to provide concurrent 64-Byte transfers from memory-mapped IO regions.  This is not needed for system memory regions because the WriteBack memory type already provides 64-Byte transfers and concurrency.

The hardware should support using the WC type for system memory, but this is not a target that an operating system is likely to be set up for.