s390x/tcg: Hide IEEE underflows in some scenarios

IEEE underflows are not reported when the mask bit is off and we don't
also have an inexact exception.

z14 PoP, 9-20, "IEEE Underflow":
    An IEEE-underflow exception is recognized for an
    IEEE target when the tininess condition exists and
    either: (1) the IEEE-underflow mask bit in the FPC
    register is zero and the result value is inexact, or (2)
    the IEEE-underflow mask bit in the FPC register is
    one.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190218122710.23639-6-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/target/s390x/fpu_helper.c b/target/s390x/fpu_helper.c
index dcad9c367a..64efab72a4 100644
--- a/target/s390x/fpu_helper.c
+++ b/target/s390x/fpu_helper.c
@@ -64,6 +64,19 @@ static void handle_exceptions(CPUS390XState *env, uintptr_t retaddr)
     s390_exc = s390_softfloat_exc_to_ieee(qemu_exc);
 
     /*
+     * IEEE-Underflow exception recognition exists if a tininess condition
+     * (underflow) exists and
+     * - The mask bit in the FPC is zero and the result is inexact
+     * - The mask bit in the FPC is one
+     * So tininess conditions that are not inexact don't trigger any
+     * underflow action in case the mask bit is not one.
+     */
+    if (!(s390_exc & S390_IEEE_MASK_INEXACT) &&
+        !((env->fpc >> 24) & S390_IEEE_MASK_UNDERFLOW)) {
+        s390_exc &= ~S390_IEEE_MASK_UNDERFLOW;
+    }
+
+    /*
      * FIXME:
      * 1. Right now, all inexact conditions are inidicated as
      *    "truncated" (0) and never as "incremented" (1) in the DXC.