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Date:	Mon, 07 Feb 2011 09:11:35 -0500
From:	Zachary Amsden <zamsden@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

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.

Cheers,

Zach
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