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IO memory emulation

Thanks to Greg Lonnon, UML (as of 2.4.6-4um) has support for I/O memory emulation. This allows a host file to be specified as an I/O region on the kernel command line. That file will be mapped into UML's kernel address space where a driver can locate it and do whatever it wants with the memory, including providing an interface to it for UML processes to use.
Driver interface
Specifying an iomem region on the kernel command line is done as follows:
The name is defined by the driver that will use the I/O area and the file is the name of the host file that will be mapped in.

The driver finds its I/O area by calling find_iomem

unsigned long find_iomem(char *driver, unsigned long *len_out)

which returns the address of the beginning of the area and passes its length out in len_out. The driver argument is the name of the I/O area as defined on the command line.

Once the driver has located its I/O area, it can do whatever it wants with it, including

  • providing a file interface to it
  • allowing processes to mmap it into their own address spaces
  • use it as a backing store for some other kind of interface, such as an X server using it as a framebuffer and providing an X protocol interface to it
The mmapper driver
arch/um/drivers/mmapper_kern.c contains a simple example driver which locates the iomem area named "mmapper" and makes it available for mapping through /dev/mmapper. Here is a little program which will mmap the /dev/mmapper device and print its contents.

To run this driver, compile it into the kernel by turning on CONFIG_MMAPPER, and assign a file to it by putting

on the command line.
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