linux-kernel - Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw

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Date:	Mon, 07 Feb 2011 16:00:00 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Zachary Amsden <zamsden@...hat.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 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.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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