sbi spec uses a6, a7 as sbicall id, so we need to set a6 to 0

os/src/sbi.rs

@@ -17,6 +17,7 @@ fn sbi_call(which: usize, arg0: usize, arg1: usize, arg2: usize) -> usize {
     let mut ret;
     unsafe {
         core::arch::asm!(
+            "li x16, 0",
             "ecall",
             inlateout("x10") arg0 => ret,
             in("x11") arg1,