tool: simplify pv_offset use

inittask_p_v_offset doesn't actually need to be calculated.

For ui_p_reg_end we know the size of the init task so can just use that
instead.

This change was motivated by inittask_p_v_offset causing exceptions
in debug builds of the tool (which are not released in the SDK) since it
is unsigned overflow and Rust does not panic for integer overflow in
release mode by default.

Signed-off-by: Ivan Velickovic <i.velickovic@unsw.edu.au>

Author: Ivan-Velickovic

tool/microkit/src/loader.rs

@@ -200,7 +200,6 @@ impl<'a> Loader<'a> {
 
         // Compute an available physical memory segment large enough to house the initial task (CapDL initialiser with spec)
         // that is after the kernel window.
-        let inittask_p_v_offset = initial_task_vaddr_range.start - initial_task_phy_base;
         let inittask_v_entry = initial_task_elf.entry;
 
         for segment in initial_task_segments.iter() {
@@ -259,10 +258,13 @@ impl<'a> Loader<'a> {
 
         let kernel_entry = kernel_elf.entry;
 
+        // initial task virt + pv_offset == initial task physical, so
+        // pv_offset == initial task physical - initial task virt
         let pv_offset = initial_task_phy_base.wrapping_sub(initial_task_vaddr_range.start);
 
         let ui_p_reg_start = initial_task_phy_base;
-        let ui_p_reg_end = initial_task_vaddr_range.end - inittask_p_v_offset;
+        let ui_p_reg_end =
+            ui_p_reg_start + (initial_task_vaddr_range.end - initial_task_vaddr_range.start);
         assert!(ui_p_reg_end > ui_p_reg_start);
 
         // This clone isn't too bad as it is just a Vec<(u64, &[u8])>