How to create a kernel virtual mapping for physical addresses.

How to create a kernel virtual mapping for physical addresses.

[brace]

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.

Any suggestions?

Don

Re: How to create a kernel virtual mapping for physical addresses.

[Arnd Bergmann]

On Saturday 09 April 2011 00:03:21 brace@beardog.cce.hp.com wrote:
> 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.

Use kmap() on the page you get from pfn_to_page().

> 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.
 
DMA mask is only about devices doing DMA to your data, not about the
kernel touching it.

	Arnd

Re: How to create a kernel virtual mapping for physical addresses.

[Arnd Bergmann]

On Thursday 21 April 2011 20:46:00 brace@beardog.cce.hp.com wrote:
> 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.

If you have highmem enabled, there probably is not enough space
to map all of this at once into the vmalloc area. If you had
that much virtual address space, you would not need highmem ;-)

> 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.

Maybe you have overwritten a kernel module and the kernel now
tries to execute within the mapped area?

> What should I be doing instead?

It's not clear at all what you are actually trying to do. Which driver
is this? If it's out-of-tree code, please post it here.

	Arnd