feat(kernel): lazy user FPU: off until first FP insn (illegal-insn trap -> zeroed regs, retry), save on dirty trap, restore on switch, off for threads without + kernel; spawntest per-thread FP check

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

@@ -1,4 +1,5 @@
 mod fast;
+mod fp;
 mod sched;
 mod vector;
 
@@ -17,6 +18,7 @@ const CAUSE_INTERRUPT_BIT: usize = 1 << 63;
 const CAUSE_S_SOFTWARE: usize = 1;
 const CAUSE_S_TIMER: usize = 5;
 const CAUSE_S_EXTERNAL: usize = 9;
+const CAUSE_ILLEGAL_INSTRUCTION: usize = 2;
 const CAUSE_ECALL_U: usize = 8;
 /// Set in `sstatus` when the trap came from supervisor mode.
 const SSTATUS_SPP: usize = 1 << 8;
@@ -29,6 +31,8 @@ pub fn init() {
         let addr = trap_vector as *const () as usize;
         core::arch::asm!("csrw stvec, {0}", in(reg) addr);
         core::arch::asm!("csrw sscratch, zero");
+        // The kernel never uses the FPU; user threads get it lazily.
+        core::arch::asm!("csrc sstatus, {0}", in(reg) 3usize << 13);
         // SIE stays clear; interrupts are enabled only by sret into userspace.
         let sie_bits = (1usize << 5) | (1usize << 9) | (1usize << 1);
         core::arch::asm!("csrs sie, {0}", in(reg) sie_bits);
@@ -90,6 +94,14 @@ extern "C" fn trap_handler_rust(frame: &mut TrapFrame) {
     }
     crate::smp::service_ipi();
     crate::smp::lock();
+    // The thread's FP registers are saved before anything can switch away.
+    if let Some(ctx) = kern()
+        .current_thread()
+        .and_then(|t| kern().thread_context_mut(t))
+    {
+        fp::save_if_dirty(frame, ctx);
+    }
+    fp::kernel_off();
     if scause & CAUSE_INTERRUPT_BIT != 0 {
         handle_interrupt(frame, scause & !CAUSE_INTERRUPT_BIT);
     } else {
@@ -163,6 +175,11 @@ fn handle_exception(frame: &mut TrapFrame, scause: usize) {
                 }
             }
         }
+        CAUSE_ILLEGAL_INSTRUCTION
+            if kern()
+                .current_thread()
+                .and_then(|t| kern().thread_context_mut(t))
+                .is_some_and(|ctx| fp::first_use(frame, ctx)) => {}
         _ => {
             let uart = crate::hw::uart::Uart::new();
             uart.write_str("FAULT: scause=");