target/ppc: Fix fallback to MFSS for MFFS* instructions on pre 3.0 ISAs

The following commits changed the code such that the fallback to MFSS for MFFSCRN,
MFFSCRNI, MFFSCE and MFFSL on pre 3.0 ISAs was removed and became an illegal instruction:

  bf8adfd - target/ppc: Move mffscrn[i] to decodetree
  394c2e2 - target/ppc: Move mffsce to decodetree
  3e5bce7 - target/ppc: Move mffsl to decodetree

The hardware will handle them as a MFFS instruction as the code did previously.
This means applications that were segfaulting under qemu when encountering these
instructions which is used in glibc libm functions for example.

The fallback for MFFSCDRN and MFFSCDRNI added in a later patch was also missing.

This patch restores the fallback to MFSS for these instructions on pre 3.0s ISAs
as the hardware decoder would, fixing the segfaulting libm code. It doesn't have
the fallback for 3.0 onwards to match hardware behaviour.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Reviewed-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230510111913.1718734-1-richard.purdie@linuxfoundation.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

target/ppc/insn32.decode

@@ -390,13 +390,19 @@ SETNBCR         011111 ..... ..... ----- 0111100000 -   @X_bi
 
 ### Move To/From FPSCR
 
-MFFS            111111 ..... 00000 ----- 1001000111 .   @X_t_rc
-MFFSCE          111111 ..... 00001 ----- 1001000111 -   @X_t
-MFFSCRN         111111 ..... 10110 ..... 1001000111 -   @X_tb
-MFFSCDRN        111111 ..... 10100 ..... 1001000111 -   @X_tb
-MFFSCRNI        111111 ..... 10111 ---.. 1001000111 -   @X_imm2
-MFFSCDRNI       111111 ..... 10101 --... 1001000111 -   @X_imm3
-MFFSL           111111 ..... 11000 ----- 1001000111 -   @X_t
+{
+  # Before Power ISA v3.0, MFFS bits 11~15 were reserved and should be ignored
+  MFFS_ISA207     111111 ..... ----- ----- 1001000111 .   @X_t_rc
+  [
+    MFFS            111111 ..... 00000 ----- 1001000111 .   @X_t_rc
+    MFFSCE          111111 ..... 00001 ----- 1001000111 -   @X_t
+    MFFSCRN         111111 ..... 10110 ..... 1001000111 -   @X_tb
+    MFFSCDRN        111111 ..... 10100 ..... 1001000111 -   @X_tb
+    MFFSCRNI        111111 ..... 10111 ---.. 1001000111 -   @X_imm2
+    MFFSCDRNI       111111 ..... 10101 --... 1001000111 -   @X_imm3
+    MFFSL           111111 ..... 11000 ----- 1001000111 -   @X_t
+  ]
+}
 
 ### Decimal Floating-Point Arithmetic Instructions
 

target/ppc/translate/fp-impl.c.inc

@@ -568,6 +568,22 @@ static void store_fpscr_masked(TCGv_i64 fpscr, uint64_t clear_mask,
     gen_helper_store_fpscr(cpu_env, fpscr_masked, st_mask);
 }
 
+static bool trans_MFFS_ISA207(DisasContext *ctx, arg_X_t_rc *a)
+{
+    if (!(ctx->insns_flags2 & PPC2_ISA300)) {
+        /*
+         * Before Power ISA v3.0, MFFS bits 11~15 were reserved, any instruction
+         * with OPCD=63 and XO=583 should be decoded as MFFS.
+         */
+        return trans_MFFS(ctx, a);
+    }
+    /*
+     * For Power ISA v3.0+, return false and let the pattern group
+     * select the correct instruction.
+     */
+    return false;
+}
+
 static bool trans_MFFS(DisasContext *ctx, arg_X_t_rc *a)
 {
     REQUIRE_FPU(ctx);
@@ -584,7 +600,6 @@ static bool trans_MFFSCE(DisasContext *ctx, arg_X_t *a)
 {
     TCGv_i64 fpscr;
 
-    REQUIRE_INSNS_FLAGS2(ctx, ISA300);
     REQUIRE_FPU(ctx);
 
     gen_reset_fpstatus();
@@ -597,7 +612,6 @@ static bool trans_MFFSCRN(DisasContext *ctx, arg_X_tb *a)
 {
     TCGv_i64 t1, fpscr;
 
-    REQUIRE_INSNS_FLAGS2(ctx, ISA300);
     REQUIRE_FPU(ctx);
 
     t1 = tcg_temp_new_i64();
@@ -614,7 +628,6 @@ static bool trans_MFFSCDRN(DisasContext *ctx, arg_X_tb *a)
 {
     TCGv_i64 t1, fpscr;
 
-    REQUIRE_INSNS_FLAGS2(ctx, ISA300);
     REQUIRE_FPU(ctx);
 
     t1 = tcg_temp_new_i64();
@@ -631,7 +644,6 @@ static bool trans_MFFSCRNI(DisasContext *ctx, arg_X_imm2 *a)
 {
     TCGv_i64 t1, fpscr;
 
-    REQUIRE_INSNS_FLAGS2(ctx, ISA300);
     REQUIRE_FPU(ctx);
 
     t1 = tcg_temp_new_i64();
@@ -647,7 +659,6 @@ static bool trans_MFFSCDRNI(DisasContext *ctx, arg_X_imm3 *a)
 {
     TCGv_i64 t1, fpscr;
 
-    REQUIRE_INSNS_FLAGS2(ctx, ISA300);
     REQUIRE_FPU(ctx);
 
     t1 = tcg_temp_new_i64();
@@ -661,7 +672,6 @@ static bool trans_MFFSCDRNI(DisasContext *ctx, arg_X_imm3 *a)
 
 static bool trans_MFFSL(DisasContext *ctx, arg_X_t *a)
 {
-    REQUIRE_INSNS_FLAGS2(ctx, ISA300);
     REQUIRE_FPU(ctx);
 
     gen_reset_fpstatus();