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

["Cédric Le Goater" <clg@kaod.org>]

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 <chigot@adacore.com>
> Fixed: 8e0a5ac87800 ("hw/ppc: Avoid decrementer rounding errors")

LGTM,

Reviewed-by: Cédric Le Goater <clg@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;