Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

[Zachary Amsden]

On 02/07/2011 10:00 AM, Jan Kiszka wrote:
> On 2011-02-07 15:11, Zachary Amsden wrote:
>    
> > On 02/07/2011 06:35 AM, Jan Kiszka wrote:
> >      
> > > On 2011-02-04 22:03, Zachary Amsden wrote:
> > >
> > >        
> > > > On 02/04/2011 04:49 AM, Jan Kiszka wrote:
> > > >
> > > >          
> > > > > Code under this lock requires non-preemptibility. Ensure this also over
> > > > > -rt by converting it to raw spinlock.
> > > > >
> > > > >
> > > > >            
> > > > Oh dear, I had forgotten about that.  I believe kvm_lock might have the
> > > > same assumption in a few places regarding clock.
> > > >
> > > >          
> > > I only found a problematic section in kvmclock_cpufreq_notifier. Didn't
> > > see this during my tests as I have CPUFREQ disabled in my .config.
> > >
> > > We may need something like this as converting kvm_lock would likely be
> > > overkill:
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 36f54fb..971ee0d 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -4530,7 +4530,7 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va
> > >    	struct cpufreq_freqs *freq = data;
> > >    	struct kvm *kvm;
> > >    	struct kvm_vcpu *vcpu;
> > > -	int i, send_ipi = 0;
> > > +	int i, me, send_ipi = 0;
> > >
> > >    	/*
> > >    	 * We allow guests to temporarily run on slowing clocks,
> > > @@ -4583,9 +4583,11 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va
> > >    		kvm_for_each_vcpu(i, vcpu, kvm) {
> > >    			if (vcpu->cpu != freq->cpu)
> > >    				continue;
> > > +			me = get_cpu();
> > >    			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> > > -			if (vcpu->cpu != smp_processor_id())
> > > +			if (vcpu->cpu != me)
> > >    				send_ipi = 1;
> > > +			put_cpu();
> > >    		}
> > >    	}
> > >    	spin_unlock(&kvm_lock);
> > >
> > > Jan
> > >
> > >
> > >        
> > That looks like a good solution, and I do believe that is the only place
> > the lock is used in that fashion - please add a comment though in the
> > giant comment block above that preemption protection is needed for RT.
> > Also, gcc should catch this, but moving the me variable into the
> > kvm_for_each_vcpu loop should allow for better register allocation.
> >
> > The only other thing I can think of is that RT lock preemption may break
> > some of the CPU initialization semantics enforced by kvm_lock if you
> > happen to get a hotplug event just as the module is loading.  That
> > should be rare, but if it is indeed a bug, it would be nice to fix, it
> > would be a panic for sure not to initialize VMX.
> >      
> Hmm, is a cpu hotplug notifier allowed to run sleepy code? Can't
> imagine. So we already have a strong reason to convert kvm_lock to a
> raw_spinlock which obsoletes the above workaround.
>    

I don't know as it is allowed to sleep, it doesn't call any sleeping 
functions to my knowledge.  What worries me in the RT case is that the 
spinlock acquired for hardware_enable might be preempted and run on 
another CPU, which obviously isn't what you want.

Zach
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