linux-kernel - Re: [PATCH 04/18] Make cpu_tsc

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Date:	Mon, 19 Jul 2010 10:06:13 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	KVM <kvm@...r.kernel.org>, Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/18] Make cpu_tsc_khz updates use local CPU

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.
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