convert the virtual address of malloc to physical address in user space

Question

I have the IPCORE which needs the physical address to communicate with the RAM. I know that When I do MALLOC in the user space it allocates the portion of memory. Now all I need to know is the address of this allocated memory. So that I can assign it to the IPCORE and let it perform an operation on it.

I hope there will be some way to do it in user space. I am not good in drivers in kernel space.

Thanks

Answer 1

A userspace program works as if it is a single program working on a machine. The program egotistically occupies all the entire address space without distinguishing between virtual and physical memory. The Virtual Memory is entirely managed by the kernel transparently to the program. If you don't have a special API for interacting with the kernel, you can never know whether you are running in a virtualized environment or on a bare metal.

So the answer depends on the OS you are using.

• In Linux, you can interact with the kernel via proc filesystem interface. All mappings of a process are located at /proc/$PID/pagemap.

  The binary format of the pagemap is described here: https://www.kernel.org/doc/Documentation/vm/pagemap.txt

  And here is an article with a particular example how to parse it: http://fivelinesofcode.blogspot.ru/2014/03/how-to-translate-virtual-to-physical.html

• As for Windows,

  according to this SO answer: How to translate a virtual memory address to a physical address?

  You have to write a custom driver.

• Other OSes - try to find it in the documentation of your OS

Also, normally you can't communicate between kernel and user space via the shared memory without a special mechanism. This may be dangerous, because VM pages may be moved to another location in the physical memory while the application is running, or even may be moved out of the physical memory to a disk (swap or pagefile). So perhaps you need to search a more proper way for your task, or at least lock shared pages of memory.