Xen E820 memory map for domU kernels

Question

How does xen handle E820 memory map for domU kernels? In my specific problem I am trying to map Non-Volatile RAM to domU kernels. The dom0 memory map returns the following relevant parts.

100000000-17fffffff : System RAM (4GB to 6GB)

180000000-37fffffff : reserved (6GB to 14GB)

The second line corrosponds to the NVRAM which is a region from 6GB to 14GB in the dom0 kernel. How can I map this NVRAM region to the domU kernel which does not map this region at all.

Ultimately I want to the nvram region to be available in other domU VMs so any solutions or advice would be highly helpful.

P.S. :: If I attempt to write to this region from the domU kernel will Xen intercept this write operation. Actually this is just a memory region write which should not be a problem, but it might appear as a hardware access.

Answer 1

Guest domains in Xen have two different models for x86: 1. Hardware Virtual Machine (HVM) : It takes benefit of Intel VT or AMD SVM extensions to enable true virtualization on x86 platform 2. Paravirtualized (PV) : This mode adds modifications in the source code of the operating system to get rid of the x86 virtualization problems and also add performance boost to the system.

These two different models handle the E820 memory map differently. E820 memory map basically gives an OS the physical address space to operate on along with the location of I/O devices. In PV mode I/O devices are available through Xenstore. The domain builder only provides a console device during boot to the pv guest. All other I/O devices have to be mapped by the guest. The guest in this mode starts execution in protected mode instead of real mode for x86. The domain builder maps the start_info pages into the guest domain's physical address space. This start_info pages contain most of the information to initialize a kernel such as number of available pages, number of CPUs, console information, Xenstore etc. E820 memory map in this context would just consist of the number of available memory pages because BIOS is not emulated and I/O device information is provided separately through Xenstore.

On the otherhand, in HVM guest BIOS and other devices have to be emulated by Xen. This mode should support any unmodified OS, thus we cannot use the previous method. BIOS emulation is done via code borrowed from Bochs, while devices are emulated using QEMU code. Here an OS is provided with an E820 memory map, build by the domain builder. The HVM domain builder would typically pass the memory layout information to the Bochs emulator which then performs the required task.

To get hold of the NVRAM pages you will have to build a separate MMU for NVRAM. This MMU should handle all the NVM pages and allocate/free it on demand just like the RAM pages. It is a lot of work.