target/arm: Do M-profile NOCP checks early and via decodetree

For M-profile CPUs, the architecture specifies that the NOCP
exception when a coprocessor is not present or disabled should cover
the entire wide range of coprocessor-space encodings, and should take
precedence over UNDEF exceptions.  (This is the opposite of
A-profile, where checking for a disabled FPU has to happen last.)

Implement this with decodetree patterns that cover the specified
ranges of the encoding space.  There are a few instructions (VLLDM,
VLSTM, and in v8.1 also VSCCLRM) which are in copro-space but must
not be NOCP'd: these must be handled also in the new m-nocp.decode so
they take precedence.

This is a minor behaviour change: for unallocated insn patterns in
the VFP area (cp=10,11) we will now NOCP rather than UNDEF when the
FPU is disabled.

As well as giving us the correct architectural behaviour for v8.1M
and the recommended behaviour for v8.0M, this refactoring also
removes the old NOCP handling from the remains of the 'legacy
decoder' in disas_thumb2_insn(), paving the way for cleaning that up.

Since we don't currently have a v8.1M feature bit or any v8.1M CPUs,
the minor changes to this logic that we'll need for v8.1M are marked
up with TODO comments.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200803111849.13368-6-peter.maydell@linaro.org

target/arm/m-nocp.decode

@@ -0,0 +1,42 @@
+# M-profile UserFault.NOCP exception handling
+#
+#  Copyright (c) 2020 Linaro, Ltd
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2.1 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, see <http://www.gnu.org/licenses/>.
+
+#
+# This file is processed by scripts/decodetree.py
+#
+# For M-profile, the architecture specifies that NOCP UsageFaults
+# should take precedence over UNDEF faults over the whole wide
+# range of coprocessor-space encodings, with the exception of
+# VLLDM and VLSTM. (Compare v8.1M IsCPInstruction() pseudocode and
+# v8M Arm ARM rule R_QLGM.) This isn't mandatory for v8.0M but we choose
+# to behave the same as v8.1M.
+# This decode is handled before any others (and in particular before
+# decoding FP instructions which are in the coprocessor space).
+# If the coprocessor is not present or disabled then we will generate
+# the NOCP exception; otherwise we let the insn through to the main decode.
+
+{
+  # Special cases which do not take an early NOCP: VLLDM and VLSTM
+  VLLDM_VLSTM  1110 1100 001 l:1 rn:4 0000 1010 0000 0000
+  # TODO: VSCCLRM (new in v8.1M) is similar:
+  #VSCCLRM      1110 1100 1-01 1111 ---- 1011 ---- ---0
+
+  NOCP         111- 1110 ---- ---- ---- cp:4 ---- ----
+  NOCP         111- 110- ---- ---- ---- cp:4 ---- ----
+  # TODO: From v8.1M onwards we will also want this range to NOCP
+  #NOCP_8_1     111- 1111 ---- ---- ---- ---- ---- ---- cp=10
+}

target/arm/meson.build

@@ -5,6 +5,7 @@ gen = [
   decodetree.process('neon-ls.decode', extra_args: '--static-decode=disas_neon_ls'),
   decodetree.process('vfp.decode', extra_args: '--static-decode=disas_vfp'),
   decodetree.process('vfp-uncond.decode', extra_args: '--static-decode=disas_vfp_uncond'),
+  decodetree.process('m-nocp.decode', extra_args: '--static-decode=disas_m_nocp'),
   decodetree.process('a32.decode', extra_args: '--static-decode=disas_a32'),
   decodetree.process('a32-uncond.decode', extra_args: '--static-decode=disas_a32_uncond'),
   decodetree.process('t32.decode', extra_args: '--static-decode=disas_t32'),

target/arm/translate-vfp.c.inc

@@ -95,14 +95,11 @@ static inline long vfp_f16_offset(unsigned reg, bool top)
 static bool full_vfp_access_check(DisasContext *s, bool ignore_vfp_enabled)
 {
     if (s->fp_excp_el) {
-        if (arm_dc_feature(s, ARM_FEATURE_M)) {
-            gen_exception_insn(s, s->pc_curr, EXCP_NOCP, syn_uncategorized(),
-                               s->fp_excp_el);
-        } else {
-            gen_exception_insn(s, s->pc_curr, EXCP_UDEF,
-                               syn_fp_access_trap(1, 0xe, false),
-                               s->fp_excp_el);
-        }
+        /* M-profile handled this earlier, in disas_m_nocp() */
+        assert (!arm_dc_feature(s, ARM_FEATURE_M));
+        gen_exception_insn(s, s->pc_curr, EXCP_UDEF,
+                           syn_fp_access_trap(1, 0xe, false),
+                           s->fp_excp_el);
         return false;
     }
 
@@ -2842,9 +2839,14 @@ static bool trans_VLLDM_VLSTM(DisasContext *s, arg_VLLDM_VLSTM *a)
         !arm_dc_feature(s, ARM_FEATURE_V8)) {
         return false;
     }
-    /* If not secure, UNDEF. */
+    /*
+     * If not secure, UNDEF. We must emit code for this
+     * rather than returning false so that this takes
+     * precedence over the m-nocp.decode NOCP fallback.
+     */
     if (!s->v8m_secure) {
-        return false;
+        unallocated_encoding(s);
+        return true;
     }
     /* If no fpu, NOP. */
     if (!dc_isar_feature(aa32_vfp, s)) {
@@ -2863,3 +2865,33 @@ static bool trans_VLLDM_VLSTM(DisasContext *s, arg_VLLDM_VLSTM *a)
     s->base.is_jmp = DISAS_UPDATE_EXIT;
     return true;
 }
+
+static bool trans_NOCP(DisasContext *s, arg_NOCP *a)
+{
+    /*
+     * Handle M-profile early check for disabled coprocessor:
+     * all we need to do here is emit the NOCP exception if
+     * the coprocessor is disabled. Otherwise we return false
+     * and the real VFP/etc decode will handle the insn.
+     */
+    assert(arm_dc_feature(s, ARM_FEATURE_M));
+
+    if (a->cp == 11) {
+        a->cp = 10;
+    }
+    /* TODO: in v8.1M cp 8, 9, 14, 15 also are governed by the cp10 enable */
+
+    if (a->cp != 10) {
+        gen_exception_insn(s, s->pc_curr, EXCP_NOCP,
+                           syn_uncategorized(), default_exception_el(s));
+        return true;
+    }
+
+    if (s->fp_excp_el != 0) {
+        gen_exception_insn(s, s->pc_curr, EXCP_NOCP,
+                           syn_uncategorized(), s->fp_excp_el);
+        return true;
+    }
+
+    return false;
+}

target/arm/translate.c

@@ -1176,6 +1176,7 @@ static TCGv_ptr vfp_reg_ptr(bool dp, int reg)
 #define ARM_CP_RW_BIT   (1 << 20)
 
 /* Include the VFP and Neon decoders */
+#include "decode-m-nocp.c.inc"
 #include "translate-vfp.c.inc"
 #include "translate-neon.c.inc"
 
@@ -8433,6 +8434,19 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
         ARCH(6T2);
     }
 
+    if (arm_dc_feature(s, ARM_FEATURE_M)) {
+        /*
+         * NOCP takes precedence over any UNDEF for (almost) the
+         * entire wide range of coprocessor-space encodings, so check
+         * for it first before proceeding to actually decode eg VFP
+         * insns. This decode also handles the few insns which are
+         * in copro space but do not have NOCP checks (eg VLLDM, VLSTM).
+         */
+        if (disas_m_nocp(s, insn)) {
+            return;
+        }
+    }
+
     if ((insn & 0xef000000) == 0xef000000) {
         /*
          * T32 encodings 0b111p_1111_qqqq_qqqq_qqqq_qqqq_qqqq_qqqq
@@ -8481,21 +8495,7 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
         /* Coprocessor.  */
         if (arm_dc_feature(s, ARM_FEATURE_M)) {
             /* 0b111x_11xx_xxxx_xxxx_xxxx_xxxx_xxxx_xxxx */
-            if (extract32(insn, 24, 2) == 3) {
-                goto illegal_op; /* op0 = 0b11 : unallocated */
-            }
-
-            if (((insn >> 8) & 0xe) == 10 &&
-                dc_isar_feature(aa32_fpsp_v2, s)) {
-                /* FP, and the CPU supports it */
-                goto illegal_op;
-            } else {
-                /* All other insns: NOCP */
-                gen_exception_insn(s, s->pc_curr, EXCP_NOCP,
-                                   syn_uncategorized(),
-                                   default_exception_el(s));
-            }
-            break;
+            goto illegal_op;
         }
         if (((insn >> 24) & 3) == 3) {
             /* Neon DP, but failed disas_neon_dp() */

target/arm/vfp.decode

@@ -213,5 +213,3 @@ VCVT_sp_int  ---- 1110 1.11 110 s:1 .... 1010 rz:1 1.0 .... \
              vd=%vd_sp vm=%vm_sp
 VCVT_dp_int  ---- 1110 1.11 110 s:1 .... 1011 rz:1 1.0 .... \
              vd=%vd_sp vm=%vm_dp
-
-VLLDM_VLSTM  1110 1100 001 l:1 rn:4 0000 1010 0000 0000