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

1 files changed, 42 insertions, 10 deletions

diff --git a/target/arm/translate-vfp.c.inc b/target/arm/translate-vfp.c.inc
index 2d63fa0d39..d376bd1c1a 100644
--- a/target/arm/translate-vfp.c.inc
+++ b/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;
+}