s390x/tcg: Fix parts of IEEE exception handling

Many things are wrong and some parts cannot be fixed yet. Fix what we
can fix easily and add two FIXMEs:

The fpc flags are not updated in case an exception is actually injected.
Inexact exceptions have to be handled separately, as they are the only
exceptions that can coexist with underflows and overflows.

I reread the horribly complicated chapters in the PoP at least 5 times
and hope I got it right.

For references:
- z14 PoP, 9-18, "IEEE Exceptions"
- z14 PoP, 19-9, Figure 19-8

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190218122710.23639-5-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

1 files changed, 32 insertions, 6 deletions

diff --git a/target/s390x/fpu_helper.c b/target/s390x/fpu_helper.c
index de02cf792d..dcad9c367a 100644
--- a/target/s390x/fpu_helper.c
+++ b/target/s390x/fpu_helper.c
@@ -63,13 +63,39 @@ static void handle_exceptions(CPUS390XState *env, uintptr_t retaddr)
     env->fpu_status.float_exception_flags = 0;
     s390_exc = s390_softfloat_exc_to_ieee(qemu_exc);
 
-    /* Install the exceptions that we raised.  */
-    env->fpc |= s390_exc << 16;
+    /*
+     * FIXME:
+     * 1. Right now, all inexact conditions are inidicated as
+     *    "truncated" (0) and never as "incremented" (1) in the DXC.
+     * 2. Only traps due to invalid/divbyzero are suppressing. Other traps
+     *    are completing, meaning the target register has to be written!
+     *    This, however will mean that we have to write the register before
+     *    triggering the trap - impossible right now.
+     */
+
+    /*
+     * invalid/divbyzero cannot coexist with other conditions.
+     * overflow/underflow however can coexist with inexact, we have to
+     * handle it separatly.
+     */
+    if (s390_exc & ~S390_IEEE_MASK_INEXACT) {
+        if (s390_exc & ~S390_IEEE_MASK_INEXACT & env->fpc >> 24) {
+            /* trap condition - inexact reported along */
+            tcg_s390_data_exception(env, s390_exc, retaddr);
+        }
+        /* nontrap condition - inexact handled differently */
+        env->fpc |= (s390_exc & ~S390_IEEE_MASK_INEXACT) << 16;
+    }
 
-    /* Send signals for enabled exceptions.  */
-    s390_exc &= env->fpc >> 24;
-    if (s390_exc) {
-        tcg_s390_data_exception(env, s390_exc, retaddr);
+    /* inexact handling */
+    if (s390_exc & S390_IEEE_MASK_INEXACT) {
+        /* trap condition - overflow/underflow _not_ reported along */
+        if (s390_exc & S390_IEEE_MASK_INEXACT & env->fpc >> 24) {
+            tcg_s390_data_exception(env, s390_exc & S390_IEEE_MASK_INEXACT,
+                                    retaddr);
+        }
+        /* nontrap condition */
+        env->fpc |= (s390_exc & S390_IEEE_MASK_INEXACT) << 16;
     }
 }