Re: [PATCH 04/18] Make cpu_tsc_khz updates use local CPU

Avi Kivity
2010-07-18T14:45:10Z

On 07/13/2010 05:25 AM, Zachary Amsden wrote:
> This simplifies much of the init code; we can now simply always
> call tsc_khz_changed, optionally passing it a new value, or letting
> it figure out the existing value (while interrupts are disabled, and
> thus, by inference from the rule, not raceful against CPU hotplug or
> frequency updates, which will issue IPIs to the local CPU to perform
> this very same task).
>
>
> @@ -893,6 +893,15 @@ static void kvm_set_time_scale(uint32_t tsc_khz, struct pvclock_vcpu_time_info *
>
> static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
>
> +static inline int kvm_tsc_changes_freq(void)
> +{
> + int cpu = get_cpu();
> + int ret = !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)&&
> + cpufreq_quick_get(cpu) != 0;
> + put_cpu();
> + return ret;
> +}
> +
> void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> {
> struct kvm *kvm = vcpu->kvm;
> @@ -937,7 +946,7 @@ void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
> }
> EXPORT_SYMBOL_GPL(guest_write_tsc);
>
> -static void kvm_write_guest_time(struct kvm_vcpu *v)
> +static int kvm_write_guest_time(struct kvm_vcpu *v)
> {
> struct timespec ts;
> unsigned long flags;
> @@ -946,24 +955,27 @@ static void kvm_write_guest_time(struct kvm_vcpu *v)
> unsigned long this_tsc_khz;
>
> if ((!vcpu->time_page))
> - return;
> -
> - this_tsc_khz = get_cpu_var(cpu_tsc_khz);
> - if (unlikely(vcpu->hv_clock_tsc_khz != this_tsc_khz)) {
> - kvm_set_time_scale(this_tsc_khz,&vcpu->hv_clock);
> - vcpu->hv_clock_tsc_khz = this_tsc_khz;
> - }
> - put_cpu_var(cpu_tsc_khz);
> + return 0;
>
> /* Keep irq disabled to prevent changes to the clock */
> local_irq_save(flags);
> kvm_get_msr(v, MSR_IA32_TSC,&vcpu->hv_clock.tsc_timestamp);
> ktime_get_ts(&ts);
> monotonic_to_bootbased(&ts);
> + this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
> local_irq_restore(flags);
>
> - /* With all the info we got, fill in the values */
> + if (unlikely(this_tsc_khz == 0)) {
> + kvm_make_request(KVM_REQ_KVMCLOCK_UPDATE, v);
> + return 1;
> + }
>

Presumably, this will spin until cpufreq writes the frequency?

> @@ -4131,9 +4138,23 @@ int kvm_fast_pio_out(struct kvm_vcpu *vcpu, int size, unsigned short port)
> }
> EXPORT_SYMBOL_GPL(kvm_fast_pio_out);
>
> -static void bounce_off(void *info)
> +static void tsc_bad(void *info)
> +{
> + __get_cpu_var(cpu_tsc_khz) = 0;
> +}
> +
> +static void tsc_khz_changed(void *data)
> {
> - /* nothing */
> + struct cpufreq_freqs *freq = data;
> + unsigned long khz = 0;
> +
> + if (data)
> + khz = freq->new;
> + else if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
> + khz = cpufreq_quick_get(raw_smp_processor_id());
> + if (!khz)
> + khz = tsc_khz;
> + __get_cpu_var(cpu_tsc_khz) = khz;
> }
>

Do we really need to cache cpufreq_quick_get()? If it's really quick,
why not just use it everywhere instead of cacheing it? Not a comment on
this patch.

--
error compiling committee.c: too many arguments to function