[PATCH v2 1/2] linux-user/s390x: signal with SIGFPE on compare-and-trap

[Jonathan Albrecht <jonathan.albrecht@linux.vnet.ibm.com>]

Currently when a compare-and-trap instruction is executed, qemu will
always raise a SIGILL signal. On real hardware, a SIGFPE is raised.

Change the PGM_DATA case in cpu_loop to follow the behavior in
linux kernel /arch/s390/kernel/traps.c.
 * Only raise SIGILL if DXC == 0
 * If DXC matches an IEEE exception, raise SIGFPE with correct si_code
 * Raise SIGFPE with si_code == 0 for everything else

When applied on 20210705210434.45824-2-iii@linux.ibm.com, this fixes
crashes in the java jdk such as the linked bug.

Buglink: https://bugs.launchpad.net/qemu/+bug/1920913
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/319
Signed-off-by: Jonathan Albrecht <jonathan.albrecht@linux.vnet.ibm.com>
---
 linux-user/s390x/cpu_loop.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/linux-user/s390x/cpu_loop.c b/linux-user/s390x/cpu_loop.c
index 22f2e89c62..6e7dfb290a 100644
--- a/linux-user/s390x/cpu_loop.c
+++ b/linux-user/s390x/cpu_loop.c
@@ -106,11 +106,13 @@ void cpu_loop(CPUS390XState *env)
 
             case PGM_DATA:
                 n = (env->fpc >> 8) & 0xff;
-                if (n == 0xff) {
-                    /* compare-and-trap */
+                if (n == 0) {
                     goto do_sigill_opn;
-                } else {
-                    /* An IEEE exception, simulated or otherwise.  */
+                }
+
+                sig = TARGET_SIGFPE;
+                if ((n & 0x03) == 0) {
+                    /* An IEEE exception, simulated or otherwise. */
                     if (n & 0x80) {
                         n = TARGET_FPE_FLTINV;
                     } else if (n & 0x40) {
@@ -121,13 +123,12 @@ void cpu_loop(CPUS390XState *env)
                         n = TARGET_FPE_FLTUND;
                     } else if (n & 0x08) {
                         n = TARGET_FPE_FLTRES;
-                    } else {
-                        /* ??? Quantum exception; BFP, DFP error.  */
-                        goto do_sigill_opn;
                     }
-                    sig = TARGET_SIGFPE;
-                    goto do_signal_pc;
+                } else {
+                    /* compare-and-trap */
+                    n = 0;
                 }
+                goto do_signal_pc;
 
             default:
                 fprintf(stderr, "Unhandled program exception: %#x\n", n);
-- 
2.31.1