linux-kernel - Re: [PATCH v4: kvm 2/4] Kill the confusing tsc_ref_khz and ref

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Date:	Fri, 09 Oct 2009 22:36:19 +0200
From:	Jan Kiszka <jan.kiszka@....de>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v4: kvm 2/4] Kill the confusing tsc_ref_khz and ref_freq
 variables.

Zachary Amsden wrote:
> On 10/08/2009 01:18 PM, Jan Kiszka wrote:
>> Zachary Amsden wrote:
>>   
>>> They are globals, not clearly protected by any ordering or locking, and
>>> vulnerable to various startup races.
>>>
>>> Instead, for variable TSC machines, register the cpufreq notifier and
>>> get
>>> the TSC frequency directly from the cpufreq machinery.  Not only is it
>>> always right, it is also perfectly accurate, as no error prone
>>> measurement
>>> is required.
>>>
>>> On such machines, when a new CPU online is brought online, it isn't
>>> clear what
>>> frequency it will start with, and it may not correspond to the
>>> reference, thus
>>> in hardware_enable we clear the cpu_tsc_khz variable to zero and make
>>> sure
>>> it is set before running on a VCPU.
>>>
>>> Signed-off-by: Zachary Amsden<zamsden@...hat.com>
>>> ---
>>>   arch/x86/kvm/x86.c |   26 ++++++++++++++++----------
>>>   1 files changed, 16 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 15d2ace..de4ce8f 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -1326,6 +1326,8 @@ out:
>>>   void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>>   {
>>>       kvm_x86_ops->vcpu_load(vcpu, cpu);
>>> +    if (unlikely(per_cpu(cpu_tsc_khz, cpu) == 0))
>>> +        per_cpu(cpu_tsc_khz, cpu) = cpufreq_quick_get(cpu);
>>>       kvm_request_guest_time_update(vcpu);
>>>   }
>>>
>>> @@ -3061,9 +3063,6 @@ static void bounce_off(void *info)
>>>       /* nothing */
>>>   }
>>>
>>> -static unsigned int  ref_freq;
>>> -static unsigned long tsc_khz_ref;
>>> -
>>>   static int kvmclock_cpufreq_notifier(struct notifier_block *nb,
>>> unsigned long val,
>>>                        void *data)
>>>   {
>>> @@ -3072,14 +3071,11 @@ static int kvmclock_cpufreq_notifier(struct
>>> notifier_block *nb, unsigned long va
>>>       struct kvm_vcpu *vcpu;
>>>       int i, send_ipi = 0;
>>>
>>> -    if (!ref_freq)
>>> -        ref_freq = freq->old;
>>> -
>>>       if (val == CPUFREQ_PRECHANGE&&  freq->old>  freq->new)
>>>           return 0;
>>>       if (val == CPUFREQ_POSTCHANGE&&  freq->old<  freq->new)
>>>           return 0;
>>> -    per_cpu(cpu_tsc_khz, freq->cpu) = cpufreq_scale(tsc_khz_ref,
>>> ref_freq, freq->new);
>>> +    per_cpu(cpu_tsc_khz, freq->cpu) = freq->new;
>>>
>>>       spin_lock(&kvm_lock);
>>>       list_for_each_entry(kvm,&vm_list, vm_list) {
>>> @@ -3120,12 +3116,14 @@ static void kvm_timer_init(void)
>>>   {
>>>       int cpu;
>>>
>>> -    for_each_possible_cpu(cpu)
>>> -        per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
>>>       if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
>>> -        tsc_khz_ref = tsc_khz;
>>>           cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block,
>>>                         CPUFREQ_TRANSITION_NOTIFIER);
>>> +        for_each_online_cpu(cpu)
>>> +            per_cpu(cpu_tsc_khz, cpu) = cpufreq_get(cpu);
>>>      
>> This doesn't build for !CONFIG_CPU_FREQ.
>>    
> 
> And did it before?

Yes, because cpufreq_get, which is undefined without CONFIG_CPU_FREQ,
did not exist so far. One may argue that this is a deficit of the
cpufreq API. However, it needs fixing.

Jan


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