riscv64, trap: fix kernel space trap returns

On riscv64 platforms, we load the kernel tp only if we've come from U
mode to reduce overhead. But we would restore the tp saved in
TrapContext even if we are returning to kernel space, which causes
problems because the default tp is zero.

We should save kernel tp register to the field in TrapContext structs
when we set privilege mode to kernel.

Signed-off-by: greatbridf <greatbridf@icloud.com>

crates/eonix_hal/src/arch/riscv64/trap/trap_context.rs

@@ -224,7 +224,15 @@ impl RawTrapContext for TrapContext {
     fn set_user_mode(&mut self, user: bool) {
         match user {
             true => self.sstatus.set_spp(SPP::User),
-            false => self.sstatus.set_spp(SPP::Supervisor),
+            false => {
+                unsafe {
+                    core::arch::asm!(
+                        "mv {}, tp",
+                        out(reg) self.regs.tp,
+                    );
+                };
+                self.sstatus.set_spp(SPP::Supervisor);
+            }
         }
     }