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

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Message-ID: <4ACF9EDF.5000808@redhat.com>
Date:	Fri, 09 Oct 2009 10:36:47 -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 10/09/2009 10:36 AM, Jan Kiszka wrote:
> 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.
>    

I'll send a patch today.  I'm going with API deficient and will fix 
that, but this means I will also require a fix for kvm-kmod :(

Zach
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