Volatile memory access for MMIO

• Use pointer::read_volatile and pointer::write_volatile.
• Never hold a reference to a location being accessed with these methods. Rust may read from (or write to, for &mut) a reference at any time.
• Use &raw to get fields of structs without creating an intermediate reference.

# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
const SOME_DEVICE_REGISTER: *mut u64 = 0x800_0000 as _;
// SAFETY: Some device is mapped at this address.
unsafe {
    SOME_DEVICE_REGISTER.write_volatile(0xff);
    SOME_DEVICE_REGISTER.write_volatile(0x80);
    assert_eq!(SOME_DEVICE_REGISTER.read_volatile(), 0xaa);
}

• Volatile access: read or write operations may have side-effects, so prevent the compiler or hardware from reordering, duplicating or eliding them.
  • If you write and then read, e.g. via a mutable reference, the compiler may assume that the value read is the same as the value just written, and not bother actually reading memory.
• Some existing crates for volatile access to hardware do hold references, but this is unsound. Whenever a reference exists, the compiler may choose to dereference it.
• Use &raw to get struct field pointers from a pointer to the struct.
• For compatibility with old versions of Rust you can use the addr_of! macro instead.