Windows Physical Direct Memory Mapping

Question

I'm a bit disappointed there is almost no discussion of this no matter where I look so I guess I'll have to ask.

I'm writing a cross platform memory bench marking application which requires direct physical address mapping rather than virtual addressing.

EDIT The solution would look something like the Linux/Unix system calls:

int fd = open("/dev/mem", O_RDONLY);
mmap(NULL, len, PROT_READ, MAP_SHARED, fd, PHYSICAL_ADDRESS_OFFSET);

which will require the kernel to either give you a virtual page mapping to the desired physical address or return that it failed. This does require supervisor privileges but that is ok.

I have seen a lot of information about shared memory and memory mapped files but all of these reside on disc and are thus not really useful when I'm trying to make a system dependent read. It is very similar to writing an IO driver although I do no need write permissions to the physical address.

This site gives an example of how to do it on a driver level using the Windows Driver Kit:

NT Insider: Sharing Memory between drivers and applications

This solution would probably require Visual Studio which currently I do not have access to. (I have downloaded the WDK api but it complained about my use of GCC for Windows).

I'm traditionally a Linux programmer so I'm hoping there might be something really simple I'm missing. Thanks in advance if you know something I don't!

Answer 1

I don't think your understanding of MAP_FIXED is correct. That tells the kernel to put the mapped memory at the given virtual address within the process address space. User-mode processes always use virtual addressing. In short, I don't think you can do what you are asking in Windows or any flavor of Unix or Linux.

Answer 2

The reason that there's not much discussion of physical memory mapping for user space apps is that user space apps can't GET physical memory mappings. Part of the definition of running in user space is that you are running in a virtual address space private to your process.

mmap with MAP_FIXED just lets you tell the system where to put the memory in your process address space. In other words, you get to specify the virtual address where the memory will show up. That's NOT a physical address in any way shape or form.