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, 0 deletions

diff --git a/target/arm/m-nocp.decode b/target/arm/m-nocp.decode
new file mode 100644
index 0000000000..7182d7d121
--- /dev/null
+++ b/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
+}