[PULL 4/9] target/m68k: add M68K_FEATURE_UNALIGNED_DATA feature

[Laurent Vivier <laurent@vivier.eu>]

From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

According to the M68040UM Appendix D the requirement for data accesses to be
word aligned is only for the 68000, 68008 and 68010 CPUs. Later CPUs from the
68020 onwards will allow unaligned data accesses but at the cost of being less
efficient.

Add a new M68K_FEATURE_UNALIGNED_DATA feature to specify that data accesses are
not required to be word aligned, and don't perform the alignment on the stack
pointer when taking an exception if this feature is not selected.

This is required because the MacOS DAFB driver attempts to call an A-trap
with a byte-aligned stack pointer during initialisation and without this the
stack pointer is off by one when the A-trap returns.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210308121155.2476-4-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
---
 target/m68k/cpu.h       | 2 ++
 target/m68k/cpu.c       | 1 +
 target/m68k/op_helper.c | 5 ++++-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/target/m68k/cpu.h b/target/m68k/cpu.h
index ce558e9b03e7..402c86c8769e 100644
--- a/target/m68k/cpu.h
+++ b/target/m68k/cpu.h
@@ -527,6 +527,8 @@ enum m68k_features {
     M68K_FEATURE_MOVEP,
     /* MOVEC insn. (from 68010) */
     M68K_FEATURE_MOVEC,
+    /* Unaligned data accesses (680[2346]0) */
+    M68K_FEATURE_UNALIGNED_DATA,
 };
 
 static inline int m68k_feature(CPUM68KState *env, int feature)
diff --git a/target/m68k/cpu.c b/target/m68k/cpu.c
index 37d2ed9dc79c..a14874b4da28 100644
--- a/target/m68k/cpu.c
+++ b/target/m68k/cpu.c
@@ -161,6 +161,7 @@ static void m68020_cpu_initfn(Object *obj)
     m68k_set_feature(env, M68K_FEATURE_CAS);
     m68k_set_feature(env, M68K_FEATURE_CHK2);
     m68k_set_feature(env, M68K_FEATURE_MSP);
+    m68k_set_feature(env, M68K_FEATURE_UNALIGNED_DATA);
 }
 
 /*
diff --git a/target/m68k/op_helper.c b/target/m68k/op_helper.c
index 5f981e5bf628..46ff81acc9f5 100644
--- a/target/m68k/op_helper.c
+++ b/target/m68k/op_helper.c
@@ -348,7 +348,10 @@ static void m68k_interrupt_all(CPUM68KState *env, int is_hw)
     cpu_m68k_set_sr(env, sr);
     sp = env->aregs[7];
 
-    sp &= ~1;
+    if (!m68k_feature(env, M68K_FEATURE_UNALIGNED_DATA)) {
+        sp &= ~1;
+    }
+
     if (cs->exception_index == EXCP_ACCESS) {
         if (env->mmu.fault) {
             cpu_abort(cs, "DOUBLE MMU FAULT\n");
-- 
2.29.2