s390x: fix cksm instruction

The cksm instruction was implemented incorrectly, rendering UDP and TCP
checksum calculation wrong, making an emulated s390x Linux guest break
in most networking operations.

This patch fixes odd end checksum calculation, takes the input register
as input for the checksum and optimizes the overflow pieces by a bit.
Signed-off-by: NAlexander Graf <agraf@suse.de>

target-s390x/op_helper.c

@@ -1731,25 +1731,15 @@ void HELPER(sqdbr)(uint32_t f1, uint32_t f2)
     env->fregs[f1].d = float64_sqrt(env->fregs[f2].d, &env->fpu_status);
 }
 
-static inline uint64_t cksm_overflow(uint64_t cksm)
-{
-    if (cksm > 0xffffffffULL) {
-        cksm &= 0xffffffffULL;
-        cksm++;
-    }
-    return cksm;
-}
-
 /* checksum */
 void HELPER(cksm)(uint32_t r1, uint32_t r2)
 {
     uint64_t src = get_address_31fix(r2);
     uint64_t src_len = env->regs[(r2 + 1) & 15];
-    uint64_t cksm = 0;
+    uint64_t cksm = (uint32_t)env->regs[r1];
 
     while (src_len >= 4) {
         cksm += ldl(src);
-        cksm = cksm_overflow(cksm);
 
         /* move to next word */
         src_len -= 4;
@@ -1760,26 +1750,24 @@ void HELPER(cksm)(uint32_t r1, uint32_t r2)
     case 0:
         break;
     case 1:
-        cksm += ldub(src);
-        cksm = cksm_overflow(cksm);
+        cksm += ldub(src) << 24;
         break;
     case 2:
-        cksm += lduw(src);
-        cksm = cksm_overflow(cksm);
+        cksm += lduw(src) << 16;
         break;
     case 3:
-        /* XXX check if this really is correct */
-        cksm += lduw(src) << 8;
-        cksm += ldub(src + 2);
-        cksm = cksm_overflow(cksm);
+        cksm += lduw(src) << 16;
+        cksm += ldub(src + 2) << 8;
         break;
     }
 
     /* indicate we've processed everything */
+    env->regs[r2] = src + src_len;
     env->regs[(r2 + 1) & 15] = 0;
 
     /* store result */
-    env->regs[r1] = (env->regs[r1] & 0xffffffff00000000ULL) | (uint32_t)cksm;
+    env->regs[r1] = (env->regs[r1] & 0xffffffff00000000ULL) |
+                    ((uint32_t)cksm + (cksm >> 32));
 }
 
 static inline uint32_t cc_calc_ltgt_32(CPUState *env, int32_t src,