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

Zachary Amsden
2010-07-19T20:06:13Z

On 07/18/2010 04:45 AM, Avi Kivity wrote:
> 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?

Only during CPU re-add; we can only get here if running before cpu
notifiers have told us about the new CPU.

>
>> @@ -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.
>

If cpufreq is compiled in, but disabled, it returns zero, so we need
some sort of logic.