[PATCH v2] target/riscv: fix wfi exception behavior

[Jose Martins <josemartins90@gmail.com>]

The wfi exception trigger behavior should take into account user mode,
hstatus.vtw, and the fact the an wfi might raise different types of
exceptions depending on various factors:

If supervisor mode is not present:

- an illegal instruction exception should be generated if user mode
executes and wfi instruction and mstatus.tw = 1.

If supervisor mode is present:

- 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.

Plus, if the hypervisor extensions are enabled:

- 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>
---
Alistair, I hope you've agreed with my argumentis for the previous version      
of the patch. As promised, I submit this version which takes into account M/U 
only harts. It checks for the presence of the RVS extension. If it is         
not present mstatus.tw takes effect over the exection of wfi in user          
mode.                                                                         

 target/riscv/cpu_bits.h  |  1 +
 target/riscv/op_helper.c | 11 ++++++++---
 2 files changed, 9 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..15982a7a33 100644
--- a/target/riscv/op_helper.c
+++ b/target/riscv/op_helper.c
@@ -173,10 +173,15 @@ target_ulong helper_mret(CPURISCVState *env, target_ulong cpu_pc_deb)
 void helper_wfi(CPURISCVState *env)
 {
     CPUState *cs = env_cpu(env);
+    bool rvs = riscv_has_ext(env, RVS);
+    bool prv_u = env->priv == PRV_U;
+    bool prv_s = env->priv == PRV_S;
 
-    if ((env->priv == PRV_S &&
-        get_field(env->mstatus, MSTATUS_TW)) ||
-        riscv_cpu_virt_enabled(env)) {
+    if (((prv_s || (!rvs && prv_u)) && get_field(env->mstatus, MSTATUS_TW)) ||
+        (rvs && prv_u && !riscv_cpu_virt_enabled(env))) {
+        riscv_raise_exception(env, RISCV_EXCP_ILLEGAL_INST, GETPC());
+    } else if (riscv_cpu_virt_enabled(env) && (prv_u ||
+        (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