Re: [Qemu-devel] [PATCH] ppcr: Avoid decrementer related kvm exits

[Alexander Graf]

On 14.10.2011, at 07:19, David Gibson wrote:

> In __cpu_ppc_store_decr(), we set up a regular timer used to trigger
> decrementer interrupts.  This is necessary to implement the decrementer
> properly under TCG, but is unnecessary under KVM (true for both Book3S-PR
> and Book3S-HV KVM variants), because the kernel handles generating and
> delivering decrementer exceptions.
> 
> Under kvm, in fact, the timer causes expensive and unnecessary exits from
> kvm to qemu.  This patch, therefore, disables setting the timer when kvm
> is in use.
> 
> Signed-off-by: Anton Blanchard <address@hidden>
> Signed-off-by: David Gibson <address@hidden>
> ---
> hw/ppc.c |   25 ++++++++++++++-----------
> 1 files changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/ppc.c b/hw/ppc.c
> index 25b59dd..87aa4e5 100644
> --- a/hw/ppc.c
> +++ b/hw/ppc.c
> @@ -658,21 +658,24 @@ static void __cpu_ppc_store_decr (CPUState *env, 
> uint64_t *nextp,

Do we ever call store_decr in the kvm case? Isn't that only called from 
emulated mtdec?

Alex

>                                   int is_excp)
> {
>     ppc_tb_t *tb_env = env->tb_env;
> -    uint64_t now, next;
> 
>     LOG_TB("%s: %08" PRIx32 " => %08" PRIx32 "\n", __func__,
>                 decr, value);
> -    now = qemu_get_clock_ns(vm_clock);
> -    next = now + muldiv64(value, get_ticks_per_sec(), tb_env->decr_freq);
> -    if (is_excp) {
> -        next += *nextp - now;
> -    }
> -    if (next == now) {
> -        next++;
> +    if (!kvm_enabled()) {
> +        uint64_t now, next;
> +
> +        now = qemu_get_clock_ns(vm_clock);
> +        next = now + muldiv64(value, get_ticks_per_sec(), tb_env->decr_freq);
> +        if (is_excp) {
> +            next += *nextp - now;
> +        }
> +        if (next == now) {
> +            next++;
> +        }
> +        *nextp = next;
> +        /* Adjust timer */
> +        qemu_mod_timer(timer, next);
>     }
> -    *nextp = next;
> -    /* Adjust timer */
> -    qemu_mod_timer(timer, next);
> 
>     /* If we set a negative value and the decrementer was positive, raise an
>      * exception.
> -- 
> 1.7.6.3
>