target/arm: Convert ERET

Pass the T5 encoding of SUBS PC, LR, #IMM through the normal SUBS path
to make it clear exactly what's happening -- we hit ALUExceptionReturn
along that path.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190904193059.26202-18-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

3 files changed, 33 insertions, 39 deletions

diff --git a/target/arm/a32.decode b/target/arm/a32.decode
index 182f2b6725..52a66dd1d5 100644
--- a/target/arm/a32.decode
+++ b/target/arm/a32.decode
@@ -211,3 +211,5 @@ BXJ              .... 0001 0010 1111 1111 1111 0010 ....      @rm
 BLX_r            .... 0001 0010 1111 1111 1111 0011 ....      @rm
 
 CLZ              .... 0001 0110 1111 .... 1111 0001 ....      @rdm
+
+ERET             ---- 0001 0110 0000 0000 0000 0110 1110
diff --git a/target/arm/t32.decode b/target/arm/t32.decode
index 67724efe4b..6236d28b99 100644
--- a/target/arm/t32.decode
+++ b/target/arm/t32.decode
@@ -218,4 +218,12 @@ CLZ              1111 1010 1011 ---- 1111 .... 1000 ....      @rdm
     MSR_v7m      1111 0011 100 0   rn:4 1000 mask:2 00 sysm:8
   }
   BXJ            1111 0011 1100 rm:4 1000 1111 0000 0000      &r
+  {
+    # At v6T2, this is the T5 encoding of SUBS PC, LR, #IMM, and works as for
+    # every other encoding of SUBS.  With v7VE, IMM=0 is redefined as ERET.
+    # The distinction between the two only matters for Hyp mode.
+    ERET         1111 0011 1101 1110 1000 1111 0000 0000
+    SUB_rri      1111 0011 1101 1110 1000 1111 imm:8 \
+                 &s_rri_rot rot=0 s=1 rd=15 rn=14
+  }
 }
diff --git a/target/arm/translate.c b/target/arm/translate.c
index 7de6949db3..9589e9bbb9 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -8503,6 +8503,27 @@ static bool trans_CLZ(DisasContext *s, arg_CLZ *a)
     return true;
 }
 
+static bool trans_ERET(DisasContext *s, arg_ERET *a)
+{
+    TCGv_i32 tmp;
+
+    if (!arm_dc_feature(s, ARM_FEATURE_V7VE)) {
+        return false;
+    }
+    if (IS_USER(s)) {
+        unallocated_encoding(s);
+        return true;
+    }
+    if (s->current_el == 2) {
+        /* ERET from Hyp uses ELR_Hyp, not LR */
+        tmp = load_cpu_field(elr_el[2]);
+    } else {
+        tmp = load_reg(s, 14);
+    }
+    gen_exception_return(s, tmp);
+    return true;
+}
+
 /*
  * Legacy decoder.
  */
@@ -8797,29 +8818,10 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
         case 0x4: /* crc32 */
             /* All done in decodetree.  Illegal ops reach here.  */
             goto illegal_op;
-        case 0x5:
-            /* Saturating addition and subtraction.  */
+        case 0x5: /* Saturating addition and subtraction.  */
+        case 0x6: /* ERET */
             /* All done in decodetree.  Reach here for illegal ops.  */
             goto illegal_op;
-        case 0x6: /* ERET */
-            if (op1 != 3) {
-                goto illegal_op;
-            }
-            if (!arm_dc_feature(s, ARM_FEATURE_V7VE)) {
-                goto illegal_op;
-            }
-            if ((insn & 0x000fff0f) != 0x0000000e) {
-                /* UNPREDICTABLE; we choose to UNDEF */
-                goto illegal_op;
-            }
-
-            if (s->current_el == 2) {
-                tmp = load_cpu_field(elr_el[2]);
-            } else {
-                tmp = load_reg(s, 14);
-            }
-            gen_exception_return(s, tmp);
-            break;
         case 7:
         {
             int imm16 = extract32(insn, 0, 4) | (extract32(insn, 8, 12) << 4);
@@ -10628,24 +10630,6 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
                     case 4: /* bxj, in decodetree */
                         goto illegal_op;
                     case 5: /* Exception return.  */
-                        if (IS_USER(s)) {
-                            goto illegal_op;
-                        }
-                        if (rn != 14 || rd != 15) {
-                            goto illegal_op;
-                        }
-                        if (s->current_el == 2) {
-                            /* ERET from Hyp uses ELR_Hyp, not LR */
-                            if (insn & 0xff) {
-                                goto illegal_op;
-                            }
-                            tmp = load_cpu_field(elr_el[2]);
-                        } else {
-                            tmp = load_reg(s, rn);
-                            tcg_gen_subi_i32(tmp, tmp, insn & 0xff);
-                        }
-                        gen_exception_return(s, tmp);
-                        break;
                     case 6: /* MRS, in decodetree */
                     case 7: /* MSR, in decodetree */
                         goto illegal_op;