Re: [PATCH v4: kvm 2/4] Kill the confusing tsc_ref_khz and ref_freq variables.

[Zachary Amsden <zamsden@xxxxxxxxxx>]

On 10/09/2009 10:47 AM, Jan Kiszka wrote:
> Zachary Amsden wrote:
>    
> > 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@xxxxxxxxxx>
> > > > > > ---
> > > > > >     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 :(
> >      
> That won't be tricky, I will queue up a fix.
>
> BTW, is the KVM prepared for cpufreq_get returning 0?
>    

That will be part of the fix, I think.

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