feat(adapter): higher-half kernel (linked at PHYS_OFFSET, boot trampoline, no low mappings), fix SBI ecall clobbers

crates/adapter-riscv/src/mmu/mod.rs

@@ -6,3 +6,20 @@ pub use page_table::{
     PAGE_SIZE, PageTable, Sv39Flags, USER_VA_END, USER_VA_START, activate, active_root, check_wx,
     create_user_root, init_kernel_page_table, map_user_page, unmap_user_page,
 };
+
+/// The kernel lives in the upper half of the address space: all of RAM and
+/// the MMIO windows appear at their physical address plus this offset (the
+/// start of Sv39's upper half), and the kernel image is linked there too,
+/// so one offset converts any kernel address. Must match linker.ld.
+pub const PHYS_OFFSET: usize = 0xFFFF_FFC0_0000_0000;
+
+/// Where the kernel sees physical address `pa`.
+pub const fn phys_to_virt(pa: usize) -> usize {
+    pa + PHYS_OFFSET
+}
+
+/// The physical address behind kernel address `va` (a static, the boot
+/// heap, a page table). Only valid for addresses in the direct map.
+pub const fn virt_to_phys(va: usize) -> usize {
+    va - PHYS_OFFSET
+}