[PATCH 2/2] target/riscv: fixes a bug against `ssamoswap` behavior in M-mode

[Deepak Gupta]

Commit f06bfe3dc38c ("target/riscv: implement zicfiss instructions") adds
`ssamoswap` instruction. `ssamoswap` takes the code-point from existing
reserved encoding (and not a zimop like other shadow stack instructions).
If shadow stack is not enabled (via xenvcfg.SSE), then `ssamoswap` must
result in an illegal instruction exception. However there is a slightly
modified behavior for M-mode.

Shadow stack are not available in M-mode and all shadow stack instructions
in M-mode exhibit zimop behavior. However, `ssamoswap` can still succeed
if MPRV=1 and MPP is non-zero (see section 2.7 of zicfiss specification).
This patch corrects that behavior for `ssamoswap`.

Fixes: f06bfe3dc38c ("target/riscv: implement zicfiss instructions")

Reported-by: Ved Shanbhogue <v...@rivosinc.com>
Signed-off-by: Deepak Gupta <de...@rivosinc.com>
---
 target/riscv/insn_trans/trans_rvzicfiss.c.inc | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/target/riscv/insn_trans/trans_rvzicfiss.c.inc 
b/target/riscv/insn_trans/trans_rvzicfiss.c.inc
index e3ebc4977c..ec016cd70f 100644
--- a/target/riscv/insn_trans/trans_rvzicfiss.c.inc
+++ b/target/riscv/insn_trans/trans_rvzicfiss.c.inc
@@ -15,6 +15,13 @@
  * You should have received a copy of the GNU General Public License along with
  * this program.  If not, see <http://www.gnu.org/licenses/>.
  */
+
+ #define REQUIRE_ZICFISS(ctx) do {       \
+    if (!ctx->cfg_ptr->ext_zicfiss) {    \
+        return false;                    \
+    }                                    \
+} while (0)
+
 static bool trans_sspopchk(DisasContext *ctx, arg_sspopchk *a)
 {
     if (!ctx->bcfi_enabled) {
@@ -77,7 +84,8 @@ static bool trans_ssrdp(DisasContext *ctx, arg_ssrdp *a)
 static bool trans_ssamoswap_w(DisasContext *ctx, arg_amoswap_w *a)
 {
     REQUIRE_A_OR_ZAAMO(ctx);
-    if (!ctx->bcfi_enabled) {
+    REQUIRE_ZICFISS(ctx);
+    if ((ctx->priv != PRV_M) && !ctx->bcfi_enabled) {
         return false;
     }
 
@@ -97,7 +105,8 @@ static bool trans_ssamoswap_d(DisasContext *ctx, 
arg_amoswap_w *a)
 {
     REQUIRE_64BIT(ctx);
     REQUIRE_A_OR_ZAAMO(ctx);
-    if (!ctx->bcfi_enabled) {
+    REQUIRE_ZICFISS(ctx);
+    if ((ctx->priv != PRV_M) && !ctx->bcfi_enabled) {
         return false;
     }
 
-- 
2.34.1