add platform dependent implementations

this allows us to get rid of traits. woo!

services/xous-swapper/src/platform/cramium/hw.rs

@@ -0,0 +1,14 @@
+/// This is an implementation for SMTs that are accessible only through a SPI
+/// register interface. The base and bounds must be translated to SPI accesses
+/// in a hardware-specific manner.
+pub struct SwapMac {
+    pub base: usize,
+    pub bounds: usize,
+}
+impl SwapMac {
+    pub fn new(base: usize, bounds: usize) -> Self { SwapMac { base, bounds } }
+
+    pub fn lookup_mac(&self, swap_page_offset: usize) -> [u8; 16] { todo!() }
+
+    pub fn store_mac(&mut self, swap_page_offset: usize, mac: &[u8; 16]) { todo!() }
+}

services/xous-swapper/src/platform/cramium/mod.rs

@@ -0,0 +1 @@
+pub mod hw;

services/xous-swapper/src/platform/mod.rs

@@ -0,0 +1,9 @@
+#[cfg(any(feature = "cramium-soc"))]
+pub mod cramium;
+#[cfg(any(feature = "cramium-soc"))]
+pub use cramium::hw::*;
+
+#[cfg(any(feature = "precursor", feature = "renode"))]
+pub mod precursor;
+#[cfg(any(feature = "precursor", feature = "renode"))]
+pub use precursor::hw::*;

services/xous-swapper/src/platform/precursor/hw.rs

@@ -0,0 +1,18 @@
+/// This defines a set of functions to get and receive MACs (message
+/// authentication codes, also referred to as the tag in AES-GCM-SIV.
+pub struct SwapMac {
+    pub macs: &'static mut [[u8; 16]],
+}
+impl SwapMac {
+    pub fn new(base: usize, bounds: usize) -> Self {
+        SwapMac {
+            // safety: this is only safe because the loader guarantees memory-mapped SMT is initialized and
+            // aligned and properly mapped into the swapper's memory space.
+            macs: unsafe { core::slice::from_raw_parts_mut(base as *mut [u8; 16], bounds as usize / 16) },
+        }
+    }
+
+    pub fn lookup_mac(&self, swap_page_offset: usize) -> [u8; 16] { todo!() }
+
+    pub fn store_mac(&mut self, swap_page_offset: usize, mac: &[u8; 16]) { todo!() }
+}

services/xous-swapper/src/platform/precursor/mod.rs

@@ -0,0 +1 @@
+pub mod hw;