hw/ppc: Fix clock update drift

The clock update logic reads the clock twice to compute the new clock
value, with a value derived from the later time subtracted from a value
derived from the earlier time. The delta causes time to be lost.

This can ultimately result in time becoming unsynchronized between CPUs
and that can cause OS lockups, timeouts, watchdogs, etc. This can be
seen running a KVM guest (that causes lots of TB updates) on a powernv
SMP machine.

Fix this by reading the clock once.

Cc: qemu-stable@nongnu.org
Fixes: dbdd250 ("Implement time-base start/stop helpers.")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-ID: <20230629020713.327745-1-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

hw/ppc/ppc.c

@@ -535,23 +535,24 @@ static inline void cpu_ppc_store_tb(ppc_tb_t *tb_env, uint64_t vmclk,
 void cpu_ppc_store_tbl (CPUPPCState *env, uint32_t value)
 {
     ppc_tb_t *tb_env = env->tb_env;
+    int64_t clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     uint64_t tb;
 
-    tb = cpu_ppc_get_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), tb_env->tb_offset);
+    tb = cpu_ppc_get_tb(tb_env, clock, tb_env->tb_offset);
     tb &= 0xFFFFFFFF00000000ULL;
-    cpu_ppc_store_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
-                     &tb_env->tb_offset, tb | (uint64_t)value);
+    cpu_ppc_store_tb(tb_env, clock, &tb_env->tb_offset, tb | (uint64_t)value);
 }
 
 static inline void _cpu_ppc_store_tbu(CPUPPCState *env, uint32_t value)
 {
     ppc_tb_t *tb_env = env->tb_env;
+    int64_t clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     uint64_t tb;
 
-    tb = cpu_ppc_get_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), tb_env->tb_offset);
+    tb = cpu_ppc_get_tb(tb_env, clock, tb_env->tb_offset);
     tb &= 0x00000000FFFFFFFFULL;
-    cpu_ppc_store_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
-                     &tb_env->tb_offset, ((uint64_t)value << 32) | tb);
+    cpu_ppc_store_tb(tb_env, clock, &tb_env->tb_offset,
+                     ((uint64_t)value << 32) | tb);
 }
 
 void cpu_ppc_store_tbu (CPUPPCState *env, uint32_t value)
@@ -584,23 +585,24 @@ uint32_t cpu_ppc_load_atbu (CPUPPCState *env)
 void cpu_ppc_store_atbl (CPUPPCState *env, uint32_t value)
 {
     ppc_tb_t *tb_env = env->tb_env;
+    int64_t clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     uint64_t tb;
 
-    tb = cpu_ppc_get_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), tb_env->atb_offset);
+    tb = cpu_ppc_get_tb(tb_env, clock, tb_env->atb_offset);
     tb &= 0xFFFFFFFF00000000ULL;
-    cpu_ppc_store_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
-                     &tb_env->atb_offset, tb | (uint64_t)value);
+    cpu_ppc_store_tb(tb_env, clock, &tb_env->atb_offset, tb | (uint64_t)value);
 }
 
 void cpu_ppc_store_atbu (CPUPPCState *env, uint32_t value)
 {
     ppc_tb_t *tb_env = env->tb_env;
+    int64_t clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     uint64_t tb;
 
-    tb = cpu_ppc_get_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), tb_env->atb_offset);
+    tb = cpu_ppc_get_tb(tb_env, clock, tb_env->atb_offset);
     tb &= 0x00000000FFFFFFFFULL;
-    cpu_ppc_store_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
-                     &tb_env->atb_offset, ((uint64_t)value << 32) | tb);
+    cpu_ppc_store_tb(tb_env, clock, &tb_env->atb_offset,
+                     ((uint64_t)value << 32) | tb);
 }
 
 uint64_t cpu_ppc_load_vtb(CPUPPCState *env)
@@ -622,14 +624,13 @@ void cpu_ppc_store_vtb(CPUPPCState *env, uint64_t value)
 void cpu_ppc_store_tbu40(CPUPPCState *env, uint64_t value)
 {
     ppc_tb_t *tb_env = env->tb_env;
+    int64_t clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     uint64_t tb;
 
-    tb = cpu_ppc_get_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
-                        tb_env->tb_offset);
+    tb = cpu_ppc_get_tb(tb_env, clock, tb_env->tb_offset);
     tb &= 0xFFFFFFUL;
     tb |= (value & ~0xFFFFFFUL);
-    cpu_ppc_store_tb(tb_env, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
-                     &tb_env->tb_offset, tb);
+    cpu_ppc_store_tb(tb_env, clock, &tb_env->tb_offset, tb);
 }
 
 static void cpu_ppc_tb_stop (CPUPPCState *env)