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

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Message-ID: <4ACF9C9B.2040006@redhat.com>
Date:	Fri, 09 Oct 2009 10:27:07 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Jan Kiszka <jan.kiszka@....de>
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.

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?
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