How to create a kernel virtual mapping for physical addresses.

Given a physical address and a length (which may span contiguous pages), how does one go about creating a virtual mapping for that address? That address may be in high memory (above 4GB) in a 32bit system.

ioremap() wants a 32bit physical address on 32bit systems. kmap() and kmap_atomic() wants a struct page which I am not sure how to get from a physical address.

If there already is a kernel mapping, then using the existing kernel mapping would be fine if I could figure out how to get it, but if there is not one, then I need to make one.

I think setting the DMA mask to 32bit would mean that I do not have to deal with highmem addresses in my particular case, but I still do not know how to get a virtual address from a physical.

Thanks for your reply.

I have a block of highmem that I want to map into my 32bit driver. This block is 256M in size.

I get the physical address and want to create a contiguous virtual buffer. I have been investigating vmap() as a possible solution.

I build up an array of pages as follows.

	pfn = physical_address >> PAGE_SHIFT
	for (i = 0; i < num_pages; i++) {
		page = pfn_to_page(pfn);
		page_array[i] = page;
		++pfn
	}

	map = vmap(page_array, num_pages, VM_IOREMAP, PAGE_KERNEL);

Using the above method, I can memset the block of memory using map as my virtual address.

However, later, I get the following error message:

kernel tried to execute NX-protected page - exploit attempt? (uid: 0)

BUG: unable to handle kernel paging request at virtual address c5000840

The address 0xc5000840 is within the range of vmap()ed memory.

What should I be doing instead?