xous-docs/memory.md

Memory Layout

Xous assumes a memory-mapped IO system. Furthermore, it assumes there is one section of "general-purpose RAM", and zero or more additional memory sections.

Memory is divided into "pages" of 4096 bytes, and are allocated based on this number. It is considered an error to request memory that isn't aligned to this address, and it is an error to request a multiple of pages that is different from this number. For example, you cannot request 4097 bytes -- you must request 8192 bytes.

Memory is allocated on a first-come, first-served basis. Physical addresses may be specified when allocating memory, in which case they are taken from that physical address. Otherwise, they are pulled from the "general-purpose RAM" section.

A process can request specific memory ranges to be allocated. For example, a uart_server might request the UART memory region be allocated so that it can handle that device and provide a service. This region cannot be re-mapped to another process until it is freed.

A process can request more memory for its heap. This will pull memory from the global pool and add it to that process' heap_size. Processes start out with a heap_size of 0, which does not include the contents of the .text or .data sections.

If a process intends to spawn multiple threads, then it must malloc that memory prior to creating the thread.

Special Virtual Memory Addresses

These addresses are statically mapped in virtual memory. They are only visible in "Supervisor" mode. However, they are globally mapped, and are available in every process.

Address	Description
0x01000000	Kernel arguments, allocation tables
0x02000000	Kernel binary image
0x04000000	Kernel data section

Memory Whitelist

Memory is kept in a whitelist. That is, when calling sys_memory_allocate(), the address is first validated against a list of known ranges. This has two major benefits:

1. It prevents attacks where memory mirrors can be reused to access another process' memory. For example, on Litex, the peripheral space is mirrored at both 0x70000000 and 0xe0000000. Without special handling, two different processes could map these two spaces and share memory.
2. We can limit the amount of memory that is needed to keep track of memory. For example, if we had generic tables that expanded every time an invalid region was accessed, then a process could use up the kernel's memory by simply requesting every possible address. In having a whitelist, we can statically allocate memory blocks to track memory usage.

Memory Tables

Each valid memory page has an associated table entry. This entry simply contains a XousPid, to indicate which process the memory block belongs to. A XousPid of 0 is invalid, and indicates the region is free. A XousPid of 1 indicates the page belongs to the kernel.

In a system with ample amounts of memory, all valid memory page would get its own memory table. However, in resource-constrained systems, a simple array is not suitable, and so a programmatic lookup table is used instead.

Kernel Arguments

There are several arguments that specify where kernel structures should go.

Memory Blocks

The kernel needs to know the range of pages. This is passed to the kernel as a list in the following form:

MBLK,$count,
$start1,$len1,$name
$start2,$len2,$name
...

The first memory range that is listed is always RAM, which is the range that will be used for dealing with sbrk and unspecified memory allocations. The name should be printable ASCII, and is primarily used for debugging.

Kernel Memory

This structure specifies how kernel memory is laid out.

Allocation Tables

Each page of memory has an entry in the allocation tables. When allocating a new page, Xous ensures that page is not currently allocated to another process. This ensures that each page of memory is only assigned to one process at a time, unless that page is handed out as shared.

Allocation tables have the following layout:

struct AllocationEntry {
    /// PID that owns this page.  `0` if this
    /// page is unallocated.
    pid: XousPid,
}

struct PageRange {
    /// A slice of all allocations within this range.
    entries: &[AllocationEntry],
}

struct PageAllocations {
    /// Each range of memory gets its own allocation table.
    /// ranges[0] is always defined as RAM,
    /// and is where memory comes from when
    /// no physical address is specified.
    ranges: &[PageRange],
}

Page Tables

Each process requires its own page table. The kernel will be mapped to a fixed offset in each process, in order to save some RAM and make context switches easier.