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 01:18:37 +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:
> 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.

Jan


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