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Re: [Qemu-devel] [PATCH] ppcr: Avoid decrementer related kvm exits
From: |
Alexander Graf |
Subject: |
Re: [Qemu-devel] [PATCH] ppcr: Avoid decrementer related kvm exits |
Date: |
Fri, 14 Oct 2011 07:30:09 +0200 |
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
>