[PATCH v2 1/3] target/m68k: don't set SSW ATC bit for physical bus errors

[Mark Cave-Ayland]

If a NuBus slot doesn't contain a card, the Quadra hardware generates a physical
bus error if the CPU attempts to access the slot address space. Both Linux and
MacOS use a separate bus error handler during NuBus accesses in order to detect
and recover when addressing empty slots.

According to the MC68040 users manual the ATC bit of the SSW is used to
distinguish between ATC faults and physical bus errors. MacOS specifically 
checks
the stack frame generated by a NuBus error and panics if the SSW ATC bit is set.

Update m68k_cpu_transaction_failed() so that the SSW ATC bit is not set if the
memory API returns MEMTX_DECODE_ERROR which will be used to indicate that an
access to an empty NuBus slot occurred.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
---
 target/m68k/op_helper.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/target/m68k/op_helper.c b/target/m68k/op_helper.c
index 202498deb5..59a6448296 100644
--- a/target/m68k/op_helper.c
+++ b/target/m68k/op_helper.c
@@ -468,7 +468,17 @@ void m68k_cpu_transaction_failed(CPUState *cs, hwaddr 
physaddr, vaddr addr,
 
     if (m68k_feature(env, M68K_FEATURE_M68040)) {
         env->mmu.mmusr = 0;
-        env->mmu.ssw |= M68K_ATC_040;
+
+        /*
+         * According to the MC68040 users manual the ATC bit of the SSW is
+         * used to distinguish between ATC faults and physical bus errors.
+         * In the case of a bus error e.g. during nubus read from an empty
+         * slot this bit should not be set
+         */
+        if (response != MEMTX_DECODE_ERROR) {
+            env->mmu.ssw |= M68K_ATC_040;
+        }
+
         /* FIXME: manage MMU table access error */
         env->mmu.ssw &= ~M68K_TM_040;
         if (env->sr & SR_S) { /* SUPERVISOR */
-- 
2.20.1