[PATCH] target/riscv: fix wfi exception behavior

[Jose Martins <josemartins90@gmail.com>]

The wfi exception trigger behavior was not taking into account the fact
that user mode is not allowed to execute wfi instructions or the effect
of the hstatus.vtw bit. It was also always generating virtual instruction
exceptions when this should only happen when the wfi instruction is
executed when the virtualization mode is enabled:

- when a wfi instruction is executed, an illegal exception should be triggered
if either the current mode is user or the mode is supervisor and mstatus.tw is
set.

- a virtual instruction exception should be raised when a wfi is executed from
virtual-user or virtual-supervisor and hstatus.vtw is set.

Signed-off-by: Jose Martins <josemartins90@gmail.com>
---
 target/riscv/cpu_bits.h  | 1 +
 target/riscv/op_helper.c | 8 +++++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/target/riscv/cpu_bits.h b/target/riscv/cpu_bits.h
index 24b24c69c5..ed8b97c788 100644
--- a/target/riscv/cpu_bits.h
+++ b/target/riscv/cpu_bits.h
@@ -436,6 +436,7 @@
 #define HSTATUS_HU           0x00000200
 #define HSTATUS_VGEIN        0x0003F000
 #define HSTATUS_VTVM         0x00100000
+#define HSTATUS_VTW          0x00200000
 #define HSTATUS_VTSR         0x00400000
 #if defined(TARGET_RISCV64)
 #define HSTATUS_VSXL        0x300000000
diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
index d55def76cf..354e39ec26 100644
--- a/target/riscv/op_helper.c
+++ b/target/riscv/op_helper.c
@@ -174,9 +174,11 @@ void helper_wfi(CPURISCVState *env)
 {
     CPUState *cs = env_cpu(env);
 
-    if ((env->priv == PRV_S &&
-        get_field(env->mstatus, MSTATUS_TW)) ||
-        riscv_cpu_virt_enabled(env)) {
+    if ((!riscv_cpu_virt_enabled(env) && env->priv == PRV_U) ||
+        (env->priv == PRV_S && get_field(env->mstatus, MSTATUS_TW))) {
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
+    } else if (riscv_cpu_virt_enabled(env) && (env->priv == PRV_U ||
+            (env->priv == PRV_S && get_field(env->hstatus, HSTATUS_VTW)))) {
         riscv_raise_exception(env, RISCV_EXCP_VIRT_INSTRUCTION_FAULT, GETPC());
     } else {
         cs->halted = 1;
-- 
2.25.1