Vectored traps for asynchrounous interrupts are optional.
The mtvec/stvec mode field is WARL and hence does not trap
if an illegal value is written. Illegal values are ignored.

Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
---
 target/riscv/op_helper.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
index f79716a..aa101cc 100644
--- a/target/riscv/op_helper.c
+++ b/target/riscv/op_helper.c
@@ -262,11 +262,10 @@ void csr_write_helper(CPURISCVState *env, target_ulong val_to_write,
         env->sepc = val_to_write;
         break;
     case CSR_STVEC:
-        if (val_to_write & 1) {
-            qemu_log_mask(LOG_UNIMP, "CSR_STVEC: vectored traps not supported");
-            goto do_illegal;
+        /* we do not support vectored traps for asynchrounous interrupts */ 
+        if ((val_to_write & 3) == 0) {
+            env->stvec = val_to_write >> 2 << 2;
         }
-        env->stvec = val_to_write >> 2 << 2;
         break;
     case CSR_SCOUNTEREN:
         env->scounteren = val_to_write;
@@ -284,11 +283,10 @@ void csr_write_helper(CPURISCVState *env, target_ulong val_to_write,
         env->mepc = val_to_write;
         break;
     case CSR_MTVEC:
-        if (val_to_write & 1) {
-            qemu_log_mask(LOG_UNIMP, "CSR_MTVEC: vectored traps not supported");
-            goto do_illegal;
+        /* we do not support vectored traps for asynchrounous interrupts */ 
+        if ((val_to_write & 3) == 0) {
+            env->mtvec = val_to_write >> 2 << 2;
         }
-        env->mtvec = val_to_write >> 2 << 2;
         break;
     case CSR_MCOUNTEREN:
         env->mcounteren = val_to_write;
-- 
2.7.0