Introduction

HAWX, like xv6, is an OS which runs on a simulated RISC-V system. The physical memory layout of that system is as specified below:

           +-------------+
0x00000000 | Unallocated | bottom 
           +-------------+
0x00001000 | BOOT ROM    |
           +-------------+
           \/\/\/\/\/\/\/
                ....
           /\/\/\/\/\/\/\
           +-------------+
0x10000000 | UART 0      |  virtio pages
           +-------------+
0x10001000 | Disk        |
           +-------------+
           \/\/\/\/\/\/\/
                ....
           /\/\/\/\/\/\/\
0x80000000 +-------------+
           | RAM         | ram memory
           |             |
           |             |
PHYSTOP    +-------------+

On top of this physical layout, the kernel maps memory using the RISC-V's memory management system. When the kernel boots, it creates a page table that maps physical memory onto their physical addresses. The kernel maps the UART and disk pages as well as all physical RAM, granting itself access to the entire contents of memory.

When we establish a user space, we will have very different memory maps for our processes! For now, we are going to create the system which allows the kernel to make use of the processor's memory mapping capabilities.

Getting Started

Take a look at the files in the kernel directory, and at main.c. Note that in this assignment, the kernel will be running with interrupts enabled, and it is possible that you will generate page fault traps. However, sometimes you will not be able to see the panic message if your page table is really messed up!

If you try to run the kernel as it is, you will see the following output:

HAWX kernel is booting

After which the kernel will stop. This is because right now, we are running without a valid page table. Memory access is not possible! Really, it is not stopped. What is happening is a series of desparate page faults because vm_init will point the kernel to an invalid page table. The CPU can do nothing but scream into the void when this happens. Let's see if we can improve the situation!

Fixing the Boot Allocation

Let's get the kernel up and running a little better, shall we? What we need to do is make sure the boot allocators are working. To do this, you need to fill in code for the following functions:

• vm_page_alloc
• vm_page_insert
• kernel_map_range

Be sure to read all of the comments, and look around the mem.c file for general hints. Once you have done this, you should at least see that you fail most of the tests.

Finishing the Assignment

To finalize the allocation and mapping functions for HAWX, you must complete the following functions:

• vm_page_free
• vm_create_pagetable
• vm_lookup
• vm_page_remove
• vm_map_range

Once you have done this, you should see the following test results:

HAWX kernel is booting

vm_page_alloc test...PASSED
vm_page_free test...PASSED
vm_lookup test...PASSED
vm_page_insert test...PASSED
vm_page_remove test...PASSED
panic: All done! (for now...)

As always, be sure to look around xv6 for inspiration, and read the comments. The hints they contain will help you!