[PATCH v4 4/4] target/ppc: Fix 64-bit decrementer

["Cédric Le Goater" <clg@kaod.org>]

The current way the mask is built can overflow with a 64-bit decrementer.
Use sextract64() to extract the signed values and remove the logic to
handle negative values which has become useless.

Cc: Luis Fernando Fujita Pires <luis.pires@eldorado.org.br>
Fixes: a8dafa525181 ("target/ppc: Implement large decrementer support for TCG")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 hw/ppc/ppc.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
index b813ef732ec1..f5d012f860af 100644
--- a/hw/ppc/ppc.c
+++ b/hw/ppc/ppc.c
@@ -821,14 +821,12 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
     CPUPPCState *env = &cpu->env;
     ppc_tb_t *tb_env = env->tb_env;
     uint64_t now, next;
-    bool negative;
+    int64_t signed_value;
+    int64_t signed_decr;
 
     /* Truncate value to decr_width and sign extend for simplicity */
-    value &= ((1ULL << nr_bits) - 1);
-    negative = !!(value & (1ULL << (nr_bits - 1)));
-    if (negative) {
-        value |= (0xFFFFFFFFULL << nr_bits);
-    }
+    signed_value = sextract64(value, 0, nr_bits);
+    signed_decr = sextract64(decr, 0, nr_bits);
 
     trace_ppc_decr_store(nr_bits, decr, value);
 
@@ -850,16 +848,16 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
      * On MSB edge based DEC implementations the MSB going from 0 -> 1 triggers
      * an edge interrupt, so raise it here too.
      */
-    if ((value < 3) ||
-        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && negative) ||
-        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && negative
-          && !(decr & (1ULL << (nr_bits - 1))))) {
+    if ((signed_value < 3) ||
+        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && signed_value < 0) ||
+        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && signed_value < 0
+          && signed_decr >= 0)) {
         (*raise_excp)(cpu);
         return;
     }
 
     /* On MSB level based systems a 0 for the MSB stops interrupt delivery */
-    if (!negative && (tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL)) {
+    if (signed_value >= 0 && (tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL)) {
         (*lower_excp)(cpu);
     }
 
-- 
2.31.1