xous-core/docs/memory.md

Memory Layout

You definitely want to refer to the Xous Book if you're trying to debug a kernel panic, before reading on here.

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. Finally, it assumes there is an MMU.

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

The last 16 megabytes of memory are reserved for use by the kernel.

Many of these pages are kernel bookkeeping on a per-process basis. For example, a process' pagetables are always mapped at 0xff400000.

The last 4 megabytes are universal across all processes, and represent the kernel's address space. These are owned by the kernel, and are available in every process in "Supervisor" mode. This should prevent the need from ever switching back to process 1.

Note that the kernel takes up a single 4 MB megapage, so it can be assigned to every process simply by mapping megapage 1023 (0xffc00000).

| Address    | Description
| ---------- | -----------
| 0x00100000 | Default entrypoint for riscv64-unknown-elf-ld (as shown by `riscv64-unknown-elf-ld --verbose`)
| 0x80000000 | Process stack top
| 0xff000000 | End of memory available to processes
| 0xff400000 | Page tables
| 0xff800000 | Process-specific data (such as root page table)
| 0xff801000 | Context data (registers, etc.)
| 0xff802000 | Return address from syscalls (never allocated)
| 0xffc00000 | Kernel arguments, allocation tables
| 0xffca0000 | Kernel LCD framebuffer page
| 0xffcb0000 | Kernel LCD CSR page
| 0xffcc0000 | Kernel GDB UART CSR page
| 0xffcd0000 | Kernel WFI CSR page
| 0xffce0000 | Kernel TRNG CSR page
| 0xffcf0000 | Supervisor UART CSR page
| 0xffd00000 | Kernel binary image and data section
| 0xfff80000 | Kernel stack top
| 0xffff0000 | "default" stack pointer (used by interrupt handlers)
| 0xfff00000 | {unused}

Note that the stack pointer is not necessarily fixed, and may be changed in a later revision.

Special Physical Memory Addresses

Address	Description
0x40000000	Bottom of battery-backed main RAM (16MiB)
0x40FFDFFF	Top of memory available to Xous
0x40FFE000	Clean suspend record - used by the system to indicate if we are coming out of a clean suspend state
0x40FFF000	Loader stack - this is corrupted by a reboot, and should not be allocated by the kernel
0x40FFFFFF	Top of battery-backed main RAM

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.

Extra Memory Regions

If additional memory is available, then it is passed in an MREx block. This is a list in the following form, with each field being 32 bits of little endian data:

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

This does not include system memory, which is passed via XArg. Instead, this is used for additional memory such as framebuffer ranges, IO ranges, or memory-mapped SPI flash. The name should be printable ASCII, and is primarily used for debugging.

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.

RISC-V RSW and V Page Table Entry Fields

The RISC-V Page Table Entry specification reserves two bits in a field called RSW. Additionally, it states that if V is 0, then the entry is invalid and all other entries are don't care. In particular, the RSW fields are for use by the kernel for its own purposes. Xous uses these to keep track of borrowed memory and pre-allocating pages.

For the purposes of Xous, PTE[8] shall have the bit name S, and PTE[9] shall have the bit name P.

P[9]	S[8]	X[3]	W[2]	R[1]	V[0]	Meaning
0	0	0	0	0	0	Page is unallocated
0	0	0	1	0	0	Invalid
0	0	1	1	0	0	Invalid
0	0	0	0	0	1	Invalid
0	0	0	1	0	1	Invalid
0	0	1	1	0	1	Invalid
0	0	x	x	x	0	Page is on-demand allocated, with permissions according to RWX
0	0	x	x	x	1	Page is allocated and valid, with permissions according to RWX
x	1	x	0	x	1	Page is immutably shared
x	1	0	0	x	0	Page is mutably shared (and is therefore unavailable)

The P[9] bit indicates whether the page was writable prior to the borrow.