Re: [PATCH] hw/ppc: fix decrementer with BookE timers

[Cédric Le Goater]

Hello Clément,

On 7/15/24 10:46, Clément Chigot wrote:

The BookE decrementer stops at 0, meaning that it won't decremented
towards "negative" values.
However, the current logic is inverted: decr is updated solely when
the resulting value would be negative.

How did you hit the issue ? which machine ? I didn't see any error
when booting Linux 6.6.3 on mpc8544ds, e500mc, e5500 and e6500.

Signed-off-by: Clément Chigot <chi...@adacore.com>
Fixed: 8e0a5ac87800 ("hw/ppc: Avoid decrementer rounding errors")

LGTM,

Reviewed-by: Cédric Le Goater <c...@redhat.com>

We have some automated tests with the ppce500 machine which it would be
interesting  to extend to have a better coverage of booke.

Thanks,

C.

---
  hw/ppc/ppc.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
index e6fa5580c0..9fc85c7de0 100644
--- a/hw/ppc/ppc.c
+++ b/hw/ppc/ppc.c
@@ -729,7 +729,9 @@ static inline int64_t __cpu_ppc_load_decr(CPUPPCState *env, 
int64_t now,
      int64_t decr;

n = ns_to_tb(tb_env->decr_freq, now);

-    if (next > n && tb_env->flags & PPC_TIMER_BOOKE) {
+
+    /* BookE timers stop when reaching 0.  */
+    if (next < n && tb_env->flags & PPC_TIMER_BOOKE) {
          decr = 0;
      } else {
          decr = next - n;